I am looking for a US source for a rechargeable (preferably gel or
sealed lead-acid) for the fin battery box of a Schempp-Hirth. I need
the extra power as well as the tail weight! Dimensions are
approximately 10 inches long, 4 inches wide and 1-1/2 inches deep.

I've found a few single-cell solutions that I could make work but
wondered if there was an off-the shelf item that would fit?

Thanks....

Mike