The foundation/basement walls in my house in Asheville, NC, have what I will call “open” concrete block walls. See the attached pictures for examples. I would like to cap or otherwise seal these off for many reasons (bugs, radon, moisture, etc), unless that’s a bad idea. The exposed area is about four inches deep. 

Assuming it’s a good idea, what do I use? I’m looking for DIY friendly. Ideally it would be a rigid surface so I can use it as a shelf, but that’s not too important. Lumber seems like a simple option, but I wonder if rigid insulation or some kind of rigid plastic would be better. I have no idea (obviously). 

Appreciate any thoughts y’all have. Thanks!