I like what you are trying and doing and
of course im interessted in the usb mod (if it isnt too much trouble).
How sturdy is the usb magnet connection?

Did your camera gain dark corner spots, because of the "spacer" or did it fit?
Did the sd card still work, after lifting the magnet?

Do you know if your samsung battery has build in protection? Did you solder it? The size seems nice. Is it 1500 mah 4.5 mm thick?

Hello pythoneye2,
the USB magnet solution are really nice and strong.
The circle connector attach very well with force and it does not have orientation. For cons I can say that it is just for powering and just a bit bulkier than the other.
The connector oval/rectangular is with all the pins (allow data transmission), it is just a bit smaller, but it is a bit less strong and to attack correctly you need to respect orientation (cannot attach upside down).

Yes, it is, the spacer makes dark angles (what I stated as tunnel vision). Not nice.. It is reduced a lot if you limit dimensions (the spacer thickness). You can test yourself, if the battery is not to much bulky and the electronic is slim too, the effect is barely visible.

The magnet acts on the magnetic switch (that normally is on the battery cover. It is the same magnet that keeps the table-holder closed in side. If you install a magnet the MMC is powered on, if you remove it, the MMC is unmounted and powered off.
In that moment, I leaved the magnet fixed with bi-adhesive tape.
Therefore each time I open the cover, or it fell apart the MMC remained powered on and working. This is dangerous if you want to hot-remove the MMC (I mean when the phone is on): it can potentially corrupt/destroy the MMC.

Almost al phone battery must have the protection. I think there are no unprotected Samsung battery: I remove it (pretty simple but keep eyes open to do not short circuit the battery). And remember to have a B-plan in case the battery break, during these kind of operation. Stay far from inflammables elements, no water (reacts with lithium), fire-extinguisher is a good add-on, but I admit I do not have.
I must say that some battery gone crazy, heating for an internal shot circuit, but they were low cost Chinese batteries. When happen keep dissipating the heat and after discharged they became safe (just safe to trash unfortunately).

Correct! That battery was 1500mA and yes a good size, the phone last a couple of days of very intensive use to me.
The battery was soldered, there is no better way to use unprotected battery. Remember to execute fast soldering junctions. If you are slow you overheat the battery, and it can damage, apart that it is risky..
And, use brand new and similar quality battery when used in parallel to assure good and safe results.

When I will have a real home, I will buy a 3D printer to print the spacer.. but till then maybe all the N900 phones will be dead..

Otherwise much better the magnetic connector.

I have done most of the hacks considered in this thread, double cell, inductive charging (Qi and Touchstone) I have rebuilt the USB port and I have installed a secondary charging board inside a Mugen bulged case. I have not tried any 3D printed solutions despite doing that as aprt of my work as I have found that 3D printed thin shell objects are very fragile and I dont feel like getting into the effort of making a injection mold for a one-off and I have zero desire to do production.

-double cells in a Mugen cover tend to flex the board and I have lost 2 N900s to the no-SIM, I might try to reball them but my current N900 works fine

-Magnet cables are a cool solution but I end up with metal dust on me, it accumulates and I have had it attract to the magnetic attach cable and put on a small pyrotechnical show resulting in a melted cable end

-Qi(see my thread) add on kits can be cool if you are up to making the data short pins, also precludes OTG which I use. Of any except for the fragility of an unprotected ribbon cable it is not a bad solution, I might revisit it now that I have a Otter case but USB into a little Sharp Zaurus style drop-in USB charging base works for now. That and without the alignment magnets of the Touchstone system Qi is hit or miss for getting an overnight charge, I don't ever even use it on the devices I own which have it factory installed.

-Touchstone, this with a charge controller inside a Mugen double cell case worked pretty good for a N900 with a ripped free USB pad, the SM soldering is a PITA and airport security gets freaked out by the bomb phone.

-FDM 3D print of a Mugen cover clone, would be fragile and all the Acetone in the world is not enough, these parts need to be injection molded or formed sheet thermoplastic.

Hello biketool,
the acetone was to remove cracks or to reduce fragility?

I am not an expert of 3D printing, but I think that choosing the right material, and increasing the thickness it could work.. Is it?

Do you know where is possible to find a Mugen cover in 2018?

by the way.. really funny the phone bomb situation.. I could had to happen to me too, but fortunately never..

the acetone 'melts' the ABS plastic by partially dissolving it into a goo, but a single or even several layer print even at high temp is still only as strong as the contact points of the squirted plastic. It just requires being realistic and often printing a multi-piece part which is glued or fastened together. Even then a printed part is (using current FDM or UV cured resin tech) not going to be as strong as a nice hot 100% fused injection molded part.
There are hacks which require several steps starting with a CAD design and FDM printing resulting in an Aluminum mold and then doing injection mold but not terribly practical for a cover which will probably require several iterations to get it right.

As for Mugen covers, they had a $5 sale on ebay areoud 2011-12 when I got mine, I am keeping it for experimental builds. I have not seen since and their website last checked a few years said not in stock.