For my EFIS, I'm using an MGL Enigma. MGL recommends the use of a 12 V, 1 ampere-hr rechargeable battery in case of electrical system failure. This battery is wired directly to the Enigma (via switch and fuse).
I went to the local "Batteries Plus" store and gave them my power requirements. For sizing, I said it had to be less than 2" tall so that it could fit up against the stiffeners under the seat sling. The resulting battery is shown below. I think the cost was around $10.
I didn't want to have to use 6 hinge pins (I'm exaggerating...) to hold a restraint/cover in place, so I opted for a wrap-over type restraint made from .025 stock as shown below. The battery pressing the restraint into the stiffener will keep the metal from bending up, and even if it did, the seat would keep it in place.
Hinges will secure each end to the stiffener yet allow easy removal when required.
1 Amphr 12v Battery & Whipped-up Restraint
Battery w/Cover Slipped Over it
Mockup of Installation Showing Hinge Pin Attachment