You just put the backer plate in a vise with rivet head against backer and swage the rivet over with proper tool or tapered punch (gently at a time hits). Don't need a helper.
You don't "flatten" a pop rivet that easy. It may bend over or who knows what. It's a tube and a mandrel you would be flattening. Precision is a little unknowledgeable on the vent/pop rivets. No one uses pop rivets on that portion. you are doing the right thing, no sweat.
The rivets, the non reassuring answer they gave and with the corner I mentioned: remind me not to have them do any vents for me.
Barrington has it right.....
"If you think you can use another type of rivet to do this step, I will add that I did also -- but the regular pop rivets will leave too much of a tail which will interfere with the operation of the vent. It is the crush rivets or nothing I am afraid."
The 55-59 frame is not as big as the one shown and can be man handled while riveting or clamp some wood support stands against both sides, sandbags, levitation, whatever.
