I have a decent -sized trunk bag but I would like to know which is the largest one on the market? (I don't want side panniers). Any info appreciated!

very suitcase like and waterproof https://www.ortlieb.com/en/Travel-Biker/ Germany.

This one's quite large:

If you want something large that is also high quality, check Axiom or Ortlieb. The Axiom Oceanweave P9 looks solid and costs under $60.

Best bet is to strap a drybag onto a rack. They come in many sizes.

If you have access to a 3D printer, you could try this:

Customizable Bike "Trunk" Container Adaptor by adamw4 - Thingiverse

You could make the rails out of plywood.