RE: Belly tank for more fuel capacity on Yellow F-15
Plastic tanks aren't bad as long as the plastic blend is flexible and thich enough to be durable. What I do not like is fiberglass tanks, I think Kevlar tanks are a direct result of people getting hosed with tank ruptures from fiberglass tanks. One Fiberglass tank is OK, you get three and four, something is gonna give on the refuel, in my experience, sooner or later.
I've had good luck with Dan Gill's tanks (Kevlar).
I've got the Gary M. Kevlar tank in my R54 too, great tank (kevlar). Replaced the Dubro plastic one.