I have been running the Painless waterproof dual battery system using an optima D34/78 (yellow top) battery which SDHQ installed in my FJ about 9 years ago. NO issues what so ever other than I recently replaced the battery with the same about a few weeks ago when Amazon had them really cheap. Painless system has rubber boots to cover the fuse area. I took one off (engine bay) so you can see and left the other on in the picture where my other fuses are located in the rear compartment so you have an idea of what the rubber boot sorta looks like.
I have used the system to run my ARB fridge, Winch (cables not in the pictures) and to jump start myself when my main battery failed on me as well as run my accessories.
Regardless of the system I would have SDHQ do the work. The attention to details like running new battery cabling system, custom mounts for the system, welded a terminal to the chassis for the negative cable and then running a separate constant 12 V line along with the regular power line for my accessories and fridge is awesome. Here are some pictures for now even though I need to do some cleaning.
Would I do anything different? Use a custom switch system vs painless switch system that came with the kit as the LED lights are too bright for where I have them located on the lower AC silver panel cover. Also the extra cost of the waterproof painless system vs regular wiring is probably not worth it. The ECU would be damaged before I have to worry about the wiring and besides I would have other problems.
here are some pictures