Rodrick, very nice design and execution, great looking sled. I too made one of these some time ago, real handy when working with twisted, cupped, and bowed lumber, saves time and effort over hand flattening. As for "……the white material is for chip control", forget it, these things find places to throw chips you didn't even know existed.
I made mine to fit my work bench width and use the bench as the bottom of the assembly, much smaller jig and easier to store in my small shop. After flattening one side, I run the board through my planner to get the second side. The volume of chips then doubles, makes for quite a mess.
Nice work on your flattening jig, enjoy.