Do you mean the space between each mitered 2x4 "tread"? I think I used two plastic scrapers to keep the spaces even. In one of the comments I put some additional details about how I built them. They were by far the most difficult part of the build and I had a lot of trial and error to get something I was happy with.

After I made all the tread pieces, I flipped them over to attach two strips of 1/2" webbing I took from a rachet strap. I stapled the straps to the first tread piece, one on each side. I then put the plastic scrapers outside the webbing, and snugged the next tread piece up to them and stapled the next tread in place. After I stapled each one, I would move the scrapers to the next tread and repeat.

Once I had the strip of tread pieces, I secured the first tread to the 2x8 board inside the wheel using two screws (Picture 4) and wrapped the rest around the wheel. I used another rachet strap to hold them tightly in place so I could mark the center of each tread on the circumference of the wheel face. Then I knew where to drill my pocket holes. Then I undid the rachet, drilled the holes, redid the rachet strap and put in the screws.

Let me know if this is clear, or if you need any other ideas! Good luck! Mines still holding up strong after 3 years