SD, you make a good point (as usual) about the use of vent filters. What I've seen in boats that come back in the fall that have seen use during the season though, is that the pumpout process draws air into the tank regularly enough that the filters seem to work pretty well. Most folks with newer boats seem to be pretty thorough about rinsing the tank after pumping out as well. Your post does remind me of a caveat I should have mentioned when suggesting the filter in the first place. The Sealand tanks we use have them already, but if I were fabricating a tank, I'd definitely add a tanksaver valve. If that filter ever jams up after overfilling the tank it probably won't clear itself...
I agree with you on building up with cloth. The whole point of epoxies are that they are stronger in laminate so you need less. In a conventional layup, mat is there primarily as a means to add density to the laminate, like filling the weave between structural cloth layers or reinforcing what will become outside corners within a female mold. To use the same technique with epoxy demonstrates an ignorance of the material's advantages.
JMHO of course.
Matt, you were asking about level indicators as well.
Again with the Sealand stuff (not a shill, it's just what I've used most), their Tankwatch
system is pretty idiot-proof. It uses a series of coloured LEDs to give you an indication of the tank's level. Their are a couple things to watch when installing them, but they work pretty well once set up. If you leave a long enough pigtail of wire (or use disconnects), it can be used as an inspection port /cleanout as well.
The Heart TankTender
system takes a bit on the part of the user and costs a fair bit more, but there's only a 1/8" air line going into the tank and the same guage can read multiple tanks containing different fluids. Oh, and it requires no electrical power to run...
Regarding your concerns about inspection plate fasteners penetrating the tanks: you could either add a plywood ring to the top of the tank, which is what it sounds like you were alluding to, or you could use anchor nuts (tee nuts) and glass right over the bottom of them.