Attached are some more pictures of the slab roller. They're pretty self-explanatory.
The strap metal is 1 1/2" by 1/8" and 1 by 1/8"
Turning a wing bolts adjusts the thickness of the slab.
The cord is nylon clothes line.
In the stirrup picture you will see 2 stirrups. I usually only use the bottom one but if I'm pulling a very long slab I will have to also step step on the higher one.
One thing that may not be clear from the pictures is that the top of the bungee hooks into a loop in the top of the rope so that it pulls down and thus pulls the back sheet of canvass up when you remove your foot.
The 17" wide rollers are from a junked package moving conveyor that the local post office scrapped and it ended up in at my scrap metal recycling source. They have PVC tube slipped over them and glued to make them a little larger.
The scrap metal and rollers were purchased at $0.35 a pound and weigh about 10 pounds total
The canvas was about $10 and there were assorted bolts and nuts for about $4.
Judy asked if different canvasses could be swapped out. If you put hooks vs just tieing the canvas to the rope, it would work well..
The basic idea of this roller could be adapted in many ways and sizes.