Here's some stuff I've done. Look at the very first project where I attach mdf as support for the gauges. When I attached the mdf, I drilled holes through the board and the plastic. I spread some jbweld on both surfaces (after severely scoreing both with 40grit) then squeezed them together till the epoxy poured out both sides. after that is cured it acts not only as a surface to surface adhesive but also as an epoxy rivet. After the job was done, I accidentally dropped it from shoulder height to the ground. No cracks at all. It been over a year now,.. flawless.
In your case, I would score the whole piece with 40, then where the clothe meets the cover, drill some holes. Place your structure. When you stretch the base clothe (fleece, cotton, whatever...) attached it to the cover where the holes are drilled with some jbweld or expoxo 88.
Also, drill two parallel
rows of holes. Spread the jb/epoxo over the first set, but when you place the cloth, only cover the first set of holes (epoxy rivet). After that dries you can hit the clothe with your resin
and start to lay down your mat. The second set of holes is for the fb resin to soak through both sides, the cover and the clothe (resin rivet).
It'll be there for life.