Scott -- Flow-rite looks similar to Trojan, but not identical like an OEM'd part. The Millenium EVO product has similar white/black fill level indicators, but looks like I have the slightly less fancy Pro-Fill that doesn't, so it's just squeeze until firm.
Stu -- good question. I've contemplated the same thing. The main reason is that the surveyor
when I bought the boat listed covering the battery terminals as one of the things to fix, so the insurance
company wants all the things he listed fixed. He was a good surveyor
overall, but it's covering a ton of stuff in a short time frame and he clearly reuses a template with lots of stock ABYC boxes to check -- e.g. he also recommended a radar
reflector and backup tiller (it's a 42' flybridge powerboat, not a sailboat, which it seems those are recommended for). So some things I have just provided an explanation as to why something doesn't need to be fixed. E.g., tell them to consider the shelf over the top to be the cover for the terminals?
Wingless -- I've thought about the DIY route
as well, but don't have access to a shop right now, nor do I want to make too big a project
out of it (designing, measuring, cutting, adjusting a custom lid).
Ann / Redcoat -- I think they might already be sprayed with some Boeshield (red anti-corrosion stuff?), and it couldn't hurt -- but it's also supposed to be reapplied every 6 months, and while it's insulative rather than conductive, it's not an insulative cover like a rubber boot -- e.g. you can still clamp down a battery cable on a terminal that's been sprayed with Boeshield and it cuts through to make a good connection (I assume similar things with dropping a wrench on it accidentally).