The center of the slotted side could be adjusted by propping the ends and lightly hammering or pressing the center out until it measures flat. Same with the tip of the second square.

Do you have a link or ebay description for these. I would like to buy a couple to put together. Thanks. keep the videos coming.

I find these 2” wide squares too much bother to attach to the table. There’s a reason the cast iron versions have just one side. Getting the fixture bolt in between the sides and tightening it is a real hassle. Not impossible, obviously, but annoying. They do make 4” wide versions which are much easier to use, but take up a lot of room on the table. The main virtue of these is the cost, which is low. A 10” high precision machined cast iron version costs $120. Some others I have cost $168 and even more. I would like these if they were made from a thicker plate so it can have just one side. And if it were 0.500 thick it could be used with standard fixture bolts for either 5/8” holes or 16mm holes. The trick to getting these correct is to make sure all the parts are tightly held together before welding. The slightest amount of looseness will cause it to be out of square, just as in assembling the table. Just remember that there is no warrantee for straightness or squareness as that’s up to your ability to get them that way.