I press a rough riser block against the sanding belt up off the table until it cleans up to make a flat surface, then I put that flat surface on the table and press the side of the rough block against the belt until it cleans up, Now I have 2 sides square to each other. Now I can go to the band saw or thickness sander to remove more to get the riser block to size.
With my table being square to the platen it's the best way for ME to square up a riser block.
All that friction from the back of the belt will wear out the platen pad in that area over time, like walt said 30.00 will fix it.
I checked my platen pad with a square a couple years ago and it needed replacing
I guess I use my edge sander different than some of yeese guy's so you may not have that problem