Posting this so somebody can stop me before I do something foolish.

Here is the plan.

I'm going to the electronics shop and find 3 conductive strips similar in width to the BP-5L battery contact.

Tape those three strips in a row and slip between where battery and power contacts meet inside.

Cut a slit in the battery cover, and run those through.

Secure a second BP-5L battery to the back, and make sure those strips contact the second battery.

My understanding is when connecting in Parallel you are doubling the capacity (amp hours) of the battery while maintaining the voltage of one of the individual batteries.

If nobody sees anything wrong with this idea, I'll press on and post a step-by-step with pictures.