Rotten battery boxes are common in pushrod Volvos. When I discovered mine had developed pin-holes years ago, I ground out the rust, took it all down to white metal and painted it completely with epoxy, then laid a layer of light fiberglass cloth in the epoxy and went over that with more epoxy and finally painted it. Still, water standing in the box is bothersome, so now I've cut lengths of heavy duty electrical cord for the battery to sit on (five pieces of 3/8" diameter insulated wire for under the battery, and the same to go between battery and the back of the box). The idea is to allow air to circulate under and behind the battery so it dries out. Water from washing or that blows back there when driving through rain may dry out from engine heat, but if the car is parked after running through rain, I doubt it evaporates from under the battery for quite a while. I'll keep an eye on mine to be sure, but I think my air spacers may help a lot.

When deciding what to use for spacers, I eliminated a bunch of stuff. Not wood because it holds moisture. Not some big brazing rod I have because it's more noble than the steel and if salty moisture were to get in there, the steel would fizz away first. I thought of sawing up a milk crate and using the plastic web panels under and behind the battery, but air wouldn't easily circulate through the waffle areas. The old electrical cord seemed okay since the copper is encapsulated in rubber and the spacing seemed adequate for air flow. I figured five strips of electrical cord running fore & aft under the battery is enough to bear and spread the battery's weight; same with behind it once the clamp is snugged down. Ideally, I'd have used nylon rod, but there wasn't any handy. Anyway, now air can blow under the battery and come out at the top rear of it, taking the moisture too. I hope.