You can glue in the firewall if you plan to never crash or don't mind more repairs if you do. Not slamming draftman, he's a good dude.
If you do plan on bouncing a bit, mounting with screws is your best bet. You'll want something on the outside of the fuse to help spread the force of the screw head on the coro. 1" wide strips of 1/8" PLY, LUAN or PVC (cut from downspout) around the outside of the fuse will work fine. Even large washers would be fine, you just want something to spread the load on the coro so it doesn't crush. You're also far ahead to cut some sections of BBQ skewers and jam them in the flutes where you plan to drill the mounting screws. If you are using the 4" fence post inside, just follow the directions for mounting the firewall.
Yes the 'rons glue between the wing skins. They mighht be double 4mm but the still hinge on one flute wall.
You've got to build a bit more beefy for the USS depending on the engine and how you plan to abuse/use it.
That being said, once you're OK with flying and repairing as needed there are many ways to build lighter gassers.