Back in the old days (late 60's), a group of us used to purchase high-density blue foam, wire cut it, then use contact cement and poster board to cover it. We'd route a groove for a spar, top and bottom, glue it in and let it dry over night, then lay a thin layer of contact cement on the foam and the "rough" side of the poster board. After the cement had a chance to set (become tacky), we'd start at the trailing edge, and carefully wrap the core with the poster board, shiny side out. Worked great!

The link I gave above was for reference purposes and technique building only. I haven't used Monokote for quite some time, but the techniques still work, no matter what heat-shrink wrapper I use. Actually, I've gone back to built-up wings with a silk and dope finish. Nostalgia? Purist? I don't know, but I love the results!