Personally I would use a stainless countersunk screw of the correct size to suit the popper.
You could also get poppers that are a screw fit.
Another possibility is to pack the back of the popper with washers to stop the alloy deforming.
Here is one of the types of popper I use.
Ebay is a good source of all things related to cover fixings, just search for " Boat cover fasteners "