It's not a good good thing, to use the lost foam method, for duct forming in the normal sense. Even if you use spackle or some other putty, unless you PVA and wax the mold, to release the putty later from the formed duct, you will have a moon scape on the inside.
For short term forming, I wrap the duct with packing tape. Then a few coats of PVA. I cut the duct off with one slice and re-assemble with a strip of glass, using CA. The plug is still usable for a second duct, if needed.
Beginning here: http://www.rcgroups.com/forums/showp...0&postcount=33
I used this method for my Cutlass. Use one wrap at a time. If you spiral the tape, it will crease. And the bare foam gets spackle and a coat of white glue or Mod-Poge puzzle sealer, before wrapping.