Inserts

Route into the foam for your inserts. Measure them out and make a drawing for later use- you will not see them when you are drilling for them later.
Cast in with epoxy. These inserts are from www.snowboardmaterials.com
Stainless Steel, 6mm, closed end, with thin plastic cover over threads to keep glue out!
Casting in area for fins. Fill up to almost full and let it set- top it off or use glass ballon-epoxy putty to fill up to foam surface. Be careful it can be hard work to sand them down to foam surface if they are overfilled.