[Daryles]

3'x3' piece of plywood,
15' of closed cell styrofoam coil
Liquid nail glue
3' 2x4
Long screws or carrage bolts

Glue the styrofoam to the plywood around the perimeter.
Lay it over the hole styrofoam down.
Have a helper hold the 2x4 over the center of the hole on the inside.
Use long screws and washers (or carrage bolts which ever reaches long enough). Drive the screws through the plywood into the 2x4 to hold it down to the roof.
Tighten the screws into the 2x4 enough to compress the styrofoam just enough to make it water tight.
Do not over tighten.