I fit the cap to the barrel where I want it, & then I fit the cap to the forestock wood where I want it. Then I accraglas the cap to the forestock & lay a small flat board about 8" long & 2" wide with a piece of saran wrap around it at the top of the forestock where the barrel would be & lightly rubber band the cap & forestock to that board as this keeps the cap level with the side of the forestock.
Then I put a long pin or wire or rod (4" long) thru the front barrel underlug hole & I take rubber bands & I go from this pin out over the end of the cap & back to the other side & this pulls the cap back tight to your cap inlet. Then I put more rubber bands around the forestock & cap to insure it is flat & now I have it back snug to the wood & up level with the forestock sides.
After it sits overnight, I remove the bands & pin & board & start working the cap & forestock wood down. After this I fit the barrel back down into the stock & insure it goes in easily & not binding & all, filing where I need to insure this.
Once this is all fitting correctly, I remove the barrel & I drill down from inside the barrel channel thru the wood & the cap & make a lil countersink into the wood & cap & put in the rivet if using such, or a larger countersink from the inside & tap the hole I drilled & insert a flathead screw from inside, cut it off on the outside & file flush to the cap. I prefer the screw over the rivet, however I am not a s PC as some.
Some guys just inlet them & rivet them but I have always glued mine as well with some good epoxy, accraglas or microbed.