You could source through a lumberyard that sells specialty plywood either HDF (similar to MDF) or Crezon The HDF routs and tools nicely but requires priming; lots of priming. The Crezon is a paper faced exterior plywood sometimes with an oiled finish (like concrete forms) which takes paint very well. Ever noticed the big signs on the highway at an exit? That is Crezon. It has to be able to weather well and this is the material they use. Hope this helps.
The space between the slab and the wall is probably left as an expansion joint. Probe the area with a screwdriver. If you bottom out between 2 and 6" my money would be on expansion joint. Material will collect in there don't let water stand in there if a freeze is imminent but other than that this isn't something to fret about.
Last edited by siriuschaos; 04-21-2011 at 01:41 PM.