Pepin,
how would you retrofit the " magic mushrooms"? I dont even know where I can get them..

I like a tight trampoline, so I would look into converting to lacing. Easiest way is probably to make a boltrope with a strip of cloth and some webbing loops. This goes into the slot in the hull. Have the trampoline re-cut and stitch webbing loops on to it so they match the loops on the part installed into the slot. Lace together and it should be solid. Expect to replace the webbing loops now and then.