I'm building a molded fiberglass cockpit rim. I've done it twice before, and both times I used mini cel foam to form the mold. I wondered if spray foam would work, with trimming and sanding. Has anyone tried this? Alternatively, can you get the cheapo type of foam used in pool noodles? Thanks.

Jim-
I can't answer any of your questions directly.
Minicell foam is too expensive for me to use for sacrificial molds - I'm cheap that way... :-) You may be able to get it more inexpensively where you are- I save it for thigh pads and hip pads, mostly.
I've used pink Owens Corning Foamular insulation board quite a bit for coaming molds and molding hatch recesses. I've also used it as a plug when building skeg boxes.
It's not very flexible, but that doesn't matter to me as I'm a believer in the 'flat plane for the cockpit coaming' as much as possible. It can be clamped in place and epoxied to the deck; the surface (even when machined) is quite fine grained so there's not much roughness left once the foam is removed.
I use a wire brush in a drill to remove the foam. The last bits dissolve with acetone.
The foam can be machined with a router (carefully, using a router table), and, of course, sawn with the bandsaw and sanded.