Rusty,
If the foam is going to be removed after the glass cures, then there's
probably something in a craft or hardware store you can use. I used home
depot Great Stuff, carved it undersize then added quick set (20 min)
sheetrock putty to get the final form for my cowl. Once dried the sheetrock
putty can be easily sanded and smoothed. If the foam is going to remain part
of you're cowl, then get the pour foam from Wicks or Spruce, or better
still, use someone's left over blue wing foam. Urethane foam has a tendancy
to shrink a little after cure.
Regards,
John Slade