Its not made for a Yamaha Tri-Z, but it is a Yamaha!
Maybe it could be modded to fit OR maybe they will build you one. $275.00 seems very reasonable if you consider material costs. It comes with a cap too. I would try and have it made bigger than 1.5 gallons, but this is set-up as a drag tank.
I hope this may help someone, I have read how much the Tri-Z folks want a top mounted tank.
I have not had a Z around since 1989ish so I forget how much room is under the tank shroud.