sytactic/expanding foams

I am searching for a method to compound a specific foam. I need it to provide consolidation pressure in a closed mold prepreg cycle.
We have used Loctites "Synspand" syntactic foam in the past. It works great, but is very expensive. We have also used 2 part urethanes, which kick-off too fast, (a modifier to the catalysis might work here...) Anyone have any experience producing their own heat expanding foam?
Our ideal cycle is 100-200% expansion at 250 degrees F over a combined ramp-up and dwell time of about 40 minutes. Low final density is preferrable. Ideal form is a rod (extruded, rolled, molded?)
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