I got those battery holders. Let’s just say I think they’d work fine in a 50 mm ping pong ball. The plastic housing itself fits in a 40 mm ball, just, but the wires coming out the ends create a problem. I might be able to cut them off and solder on perpendicular wires. Or I could go with this http://www.mouser.com/ds/2/209/EPD-200504-191955.pdf which appears to be a through hole version of the same thing. I’m not so sure why I didn’t go with that in the first place. But I’m getting the feeling direct soldering to the battery might be best, anyway.
I got the RTDs too (and the 1% fixed resistors to go with them), but haven’t tried them out yet. I’m pretty sure the RTDs will fit: