Missing fuel tank hatch

[Starling]

Hello all, it's me again -
I'm getting my list of priorities together before I tear into my boat this weekend and I have a major one for you.
Apparently the fuel tank was replaced on my 89' Tracker 20SF at some point and instead of just reusing the tank hatch they used aluminum diamond plate slapped down and screwed in. Obviously this has to go, and I need to evaluate the conditions under the plate including the tank and all that's connected to it.
My question is how would you go about fixing this: fabricate a new hatch (which I have no template or dimensions) or glass it in.
I'm thinking either 3/4" coosa or marine ply with supports across the width if I choose to enclose the tank. I'm not 100% sure what to do If it's recommended I fabricate a hatch.
-Thanks