This is a link someone gave in the archives.  This is what they used to
replace the foam on the heater flaps:

http://www.homedepot.ca/product/adhesive-foam-tape/941505

(in case that link dies over the years, here are some details that might
help someone search it out:

TAGO Adhesive Foam Tape

Model: TA78537 | Store SKU: 1000124257
)

On the right rail on that page there are several references to foam tape
that is explicitly advertised as closed-cell (e.g.
http://www.homedepot.ca/product/closed-cell-foam-tape-self-adhesive-weatherstripping/941502

TAGO Closed-Cell Foam Tape Self-Adhesive Weatherstripping

 Model: TA78626 | Internet/Cat: 941502 | Store SKU: 1000113273 )

Is the difference just that one is porous like a natural sponge and one is
formed of a bunch of closed bubbles stuck together?  I saw a lot of
recommendations that you use "open-cell foam", so I guess I'm wondering if
that's the "default foam" :).

mike