Need ideas for interior trunk panelling.

If you look in the back of some other vehicles you'll find ideas. Like In the back of a Camry there are small pieces of hardboard on each side to fill the void between trunk floor and quarter skin. Then the carpet lays right over those and the metal trunk floor.
A 90 Accord witl fold down rear seats has a huge piece of carpet that covers the back of the seats to the tail lights. I like the greyish carpet.
Now the harder part... The 75 Valiant Brogham edition had a molded plastic cover that caps from truck lid catch to both tail lights. That piece is taller that what your fish needs so you'll need to rip it. That same Valiant had fabric glued directly over the rear wheel wells and a sleeve over the filler tube. Velcro works well in a lot of places.
I would rather go to a parts yard like LKQ and get auto grade materials for nearly nothing than to go to the hardware store and pay for large pieces of new material that may not hold up.
There are backings on some house carpets that will go through paint like a rasp too.
Just food for thought. Show us what you end up doing. Good luck