I made one today.... I used a 2x4
I purchased a set of 5" casters from amazon $30 They said they were 6" total tall.... NO, they were 6.25" (6-1/4") tall.
For the control / outrigger side I used a piece of 1x4 (it was a true 1" thick)...
I tried to make the wood dolly as close to what the metal tubing dolly was like.
Here is what I think should be done.... Use a 2x6 for the control / outrigger side.... Because I think it needs the extra 1/2" height..... Mine was resting on the tie down brackets (where it says "not a step)..... I used a piece of scrap 5/8" material (not pictured) on the top of the dolly...... so hopefully the tie down brackets will not come in contact with the top of the dolly.
Total cost $50 for casters, nuts and bolts and lumber.