How would you want to use two shifts together on such a keyboard?

I use Chr as an extra shift key (see http://my.arava.co.il/~matan/770/heb/rx-44-he ) for hebrew, but also for some extra keys. If you don't need it for local characters, it is you best bet.

Some remarks:
* Shift with -+ ' gives additional three options.
* Do you need Yen and Euro symbols?
* There is ESC key (the round arrow under the light sensor).

I think what is most needed is good documentation of XKB.