Yes you would need the spray foam. The purpose of the foam insert is to allow for the spray foam to fill only the top 4 inches or so. So it doesn't just run all down the mast. You could make you own out of anything really. The small plate is used to stuff the foam down and then insert the comp tip. Once installed then spray foam will create the seal from the comp tip down. The comp tip and up has its own foam built in to seal the tip. If the spray foam is not done than it will fill the mast and good luck getting the boat righted. We just used an entire tube of marine JB weld to epoxy the tip on. It really just fills the gaps and voids. The rivits and gravity hold it on.
1981 Hobie 16 cat fever
1981 Hobie 16 double plum