I use 3mm MDF. I cut it with a packing knife, sand the edges to make it taper, then paint both sides with black acrylic to seal it. For heavy stuff, I use two layers of 3mm, with the center only having the second layer. Use a mask, MDF dust is hella bad for you.
If you glue several layers of construction card together with pva pressed under heavy books it gives good results. Foamcore tends to warp and dent.
I also use CDS a lot. Be sure to put the grooved side UP, not down or it will flake foil everywhere.