I was thinking that a thin tank underneath the seat (or part of the seat)would get some extra capacity on both LT's.
You'd either need an interconnect line or a separate filler behind the seat and a tee with a shutoff into the fuel line to make it work.
There's space for the tank to sit lower than the frame rail, but not a whole lot unless you relocated the air filter farther down (carb height is the limiting factor in fuel flow unless a transfer pump is used).
If a guy could make the tank double as a seat frame, you could theoretically get even more capacity.
Run the main tank dry, then switch over to the seat tank as a reserve.
Now that would be a hell of a sleeper mod...

Alternatively, a thin walled aluminum tank will give you extra capacity in the same place as the OEM tank.
Make the clearance tighter around the backbone, and lengthen the edges that droop around the backbone with an interconnect line between both sides (or dual petcocks feeding into a single fuel line).
Couple that with the tank being farther back, farther forward, bulging out to actually touch the plastic, and a taller filler neck...5 gal might be a reasonable capacity.
You could get even more if you weren't worried about it looking OEM by raising the top of the tank and cutting out the plastic around it, then paint the exposed portion of the tank to match the yellow plastics.
Form fitting an aluminum tank to compound curves isn't easy, and it'll cost you, but there's a lot of capacity to be had compared to the OEM tank.
Perhaps a fiberglass tank would be easier (and probably lighter)....you could use the plastics as a mold to form the top half of the fuel tank.
On the bottom half, you can lay down a thin sheet of foam padding on the backbone, then form fit the fiberglass over it and shape as needed.
Join the two halves, add a filler neck, fittings, maybe add some aluminum strips under a layer or two for added strength, and you've got a lightweight form fitting tank with extra capacity.

Another way to build a fiberglass tank...
Coat plastic and frame with release agent (or saran wrap), install the plastic over the frame, turn the frame upside down, and slowly fill the cavity with expanding foam.
This will make a form fitting cast of the tank area.
Flip the frame over and test fit the foam mock-up, add foam in areas that it didn't fill (or areas that you want extra fuel tank), sand or cut the foam as needed to shape and fit the mock-up to the quad.
Once you get the shape you want and it fits good, sand off a thin layer of foam (1/4" or so) from the entire mock-up.
Here's where it gets interesting.
Coat the foam with epoxy and let harden (think of it as a gel coat for the inside of the tank), fiberglass over the foam mock-up and let cure, then pour gas into the newly formed tank.
The gas will dissolve the foam, leaving only the fiberglass that you laid up around it.
Clean out any remains of foam, add fittings and filler neck, and you've just made a form fitting tank without a ton of work.
Learned that trick from homebuilt aircraft manufacturing...