Been working on foamers this week actually, we recieved a shipment of trailers at my job and they stack them on large foam blocks, I get 3 herters 63 size decoys from each block, I would check trailer places, tractor supply, or a store like lowes ect. for the blocks. As for shaping them, I used a drywall knife to rough out the shape based on a herters body and used a rasp to shape the rest.
It sure is messy cause the foam blocks are s course foam unlike the pink stuff but it holds together well with mastic and burlap. Only downside is it's hard to cut fine wings or details into it.
I'm using herters heads and just painting on the details when I'm done. As for a keel, I'm messing around with cedar keels or may try the rebar insert idea.