The only bendable foam board I have ever seen is the light blue insulation sheets at Lowes/Home depot. Not sure how well it would work.
Advanced Pastics in Elberta may have something.
Your other option is to make a mold by forming two tubes. One tube being the inner diameter and the other tube being the outer diameter. You then coat the surfaces that will come into contact with the foam with a mold release (a wax like product to used to prevent resin from sticking to mold). Slide the inner tube into the outer tube. Brace the inside tube crossways in several places. Put spacers between the tubes and then pour your foam mxture into the mold allowing for expansion. Don't over fill. You can always add more. Remove the spacers before foam sets and pour foam into those spots. You are going to need at least 2 inches of foam thickness so take that into account when you make your mold.
After foam cured, carefully remove the tubes. You can then lay up your resin and laminate. Roll gel coat to level and then sand and buff it out.
Let me know if you have more questions or need any help.