The acorn nuts spin onto the installed pop-rivet end just fine. I use a electric nut drive set at minimum torque and on they go.....
IMHO: I think that Scamps method is compromising the long term security of the rivets by requiring that they pull the joint together AND retain the acorn nut at the same time.
I will continue to replace any pop-rivets in fiberglass using backing washers and then threading the acorn nuts on after the fact.
Let's see.... Does anyone ever hear of pop-rivets coming loose/leaking in a Scamp