Project Another FZR project

It looks like it got dropped and accidentally stepped on.
You don't need a really high vacuum pressure seal with the diaphragm as the slide 'breathes' through the holes in the bottom.

Fuel resistant glue for plastic may be difficult to locate because the plastic itself is resistant to fuel and most solvents - methylene chloride might work, but it's 'hot' - boils off near immediately as it's so volatile

 

Here's the article on the connection between the YZR and FZR's from Yamaha I think you looking for.
https://global.yamaha-motor.com/stories/yamaha-handling/list/vol07/01.html

 

FWIW - I asked @gregt if the 2KR fuel cell and tank cover looks to suit the 3LN frame, he reckons it does - you've got both, see how it looks, it's a good idea.

Otherwise you could shorten the bracket which attached to the tank and either redrill or modify the bracket which attached to the frame to get the 3LN tank where you want it.

 

I'll see if I can find it, a guy did a great writeup and pictorial of molding an Aprilia RSV tank, simpler if you're going to do a cover as you only need to mold the top, plus once you've done the mold, you could add to the length to the mold of the cutout so it accommodates your knees.
CUSTOM!

That can be done with something called microballoons - it gets mixed with resin to make it a paste for shaping.
Good part is that it is really easy to sand.

Simply put, take a piece of ply or MDF, cut to the shape across the centre of the tank.
Tape it in place and use plasticine to make it a smooth edge for the mold half.

Mold that half.

Then remove the plywood, put a plastic film as a separator line which sits up on the plywood on the first molded half and do the second half.

All told, the most difficult part is getting the ply cut to a close conforming shape and sanding things smooth prior to either applying mold release wax or PVA mold release takes tim bit isn't actually difficult

 

Priorities, they can sometimes be elusive...