I know there is already a million sanding blocks out there. Now there is a million and one!
This sanding block is made for half sheets of the 3-2/3 x 9 inch sheets sold here in the states at least. It is very easy to replace sheets and get the sheet taught in the tool before clamping with the knob. The clamping action is firm and will not slip.
You will need to print the three .stl files in the file list in addition to one each of 1/4 inch flat washer, 1/4-20 Hex Nut and 1/4-20x 2-1/2 long hex bolt.
As I usually try to do, I included the model as a step file so you can remix this to suit your needs. For instance you could resize the holes and pockets for metric bolts or resize the sides for different sheet sizes.
To make this block really flat, I sanded the Block Clamp on a granite block while assembled to the Handle under the tension of the knob & bolt while under use. I started with 320 grit paper until I just removed very little material at the nut pocket. I then used finer paper to get it smoother.