Gonna go with DuraVent's storm collar. It's metal, has the 3" hole I'd need for the pipe and the hole is "sleeved" over the pipe, which should help create a better seal. I've previously used flat sheet and it's more challenging to keep that sealed since it would be at a right angle to the pipe.
DuraVent PelletVent 3 in. Storm Collar-3PVL-SC - The Home Depot
PelletVent is a complete venting system from DuraVent for stoves and inserts that use oil fuel or multi-fuels (including wood pellets, corn, cherry pits, switch grass, coffee husks, walnut shells, soybeans,
www.homedepot.com
