Mike,
If you are just going to use this foam board to break down sheets of plywood or other materials with your circular saw, I went a different way because I can no longer get down on my knees when working. It's actually easier than using the foam, but you could still use the foam if you wanted to. Since my shop is small, I always break down my sheet stock out in my driveway, then carry the pieces into my shop to finish cutting them on my Unisaw.
I built a cutting table so I can stand up while doing this work. It's just a banquet table sized wood frame made from 1 X 4 pine with 2 X 4 cross pieces laid flat and level with the top edge of the 1 X 4 frame. There are five of these cross pieces, one across the center, and two near each end where the metal banquet table legs get attached. The frame and cross pieces are all assembled with glue and biscuits. The only metal are the legs and the short screws that attach them. You can get a set of these legs from Northern or from Harbor Freight for about $20. To use it I set it up and lay the material to be cut on top of it, then set my saw depth to cut through the material plus about 1/4". Set and clamp my saw guide to the material and make the cut. None of the pieces fall as the cuts are completed and there are never any broken corners, so common when trying to use saw horses. When the work is complete I fold the table and store it against the other sheet stock in my wood rack. The legs fold up inside the frame so when folded the table is the thickness of the 1 X 4 frame.
Attached are photos of my first cutting table. It was made completely of 2 X 4's, and as I got older I decided to remake the top frame from 1 X 4 material to reduce it's weight (no pictures of the new lighter version). It's also slightly smaller at about 30" wide and 70" long. This also reduced the weight. I always cut over the center of the table, so the pieces don't fall when the cut is complete.
I added two small pieces of plywood (shown in the pictures), attached off center with one screw through each to one side of the table frame. I can turn these pieces so they are above the table frame and lay the table with these pieces down against the ground. Then place the sheet-to-be-cut on these, and then tilt both the sheet-to-be-cut and the table back up onto it's legs. When the table is upright, I then rotate these two pieces so they are blow the table surface and then center the sheet on the table. Doing this is easier than trying to lift the full sheets to place them onto the table.
Charley