Mr. Blodgett, One trick you might want to investigate if you are pressed into using home insulation foam for a form on a one off on a squirt boat is to use epoxy resin and hardener....which might indeed eat at your foam form. However, to prevent or minimize nibbling first make a slurry of epoxy and hardener and filler or microballoons, or woodflour....(whatever is cheapest) to a consistancy that will hang on vertical surfaces after it cures....sand and fair and continue the one off process using PVA (that's poly vinyl alcohol, available at most better boating stores) as a mold release. Happy squirting.