Does anyone make a plastic latching access panel? I've been using the iPower rechargeables for my wireless IEMs and wouldn't mind expanding them to my bass, but it seems ridiculous to have to remove screws (from wood which will eventually strip) to get to the batteries in a bass.

Fishman makes one:

http://www.elderly.com/accessories/items/FBB1.htm

Allparts has them also: http://www.allparts.com/store/electronics-capacitors-etc-ep-0929-023,Product.asp

Or you can do the magnet attachment like I do (and later found out that Dingwall also uses). It looks clean, it's strong enough and easy to remove.

And stewmac........t

http://www.stewmac.com/shop/Electronics,_pickups/Battery_clips_and_holders/Battery_Boxes.html

Any concerns with body thickness, or should they work with just about any normal bass? I play a Peavey Millennium 5 BXP AC (18V).

Hey we must be related! :)
You can also check here too:
http://www.guitarelectronics.com/category/20electronicparts.8batteryaccessories/

Or you can do the magnet attachment like I do (and later found out that Dingwall also uses). It looks clean, it's strong enough and easy to remove.

What is that magnet attachment? I have been trying to think of an easy way to access the battery, and this is one that I haven't heard of before.

I think the coolest is David King's. No hardware at all. Dovetail jointed sliding cover, like those craft show boxes.

Allparts has them also: http://www.allparts.com/store/electronics-capacitors-etc-ep-0929-023,Product.aspI used this same one to add a battery to my SX Jazz. Obviously, you need to be somewhat adept with a router to install it.

And, just so you know, it does not come with the wiring. That's a separate buy!