This design is from here, dimensions are determined by counting holes.
Only thing to be aware of is that there are two types of two hole plates. The ones that keep strut side by side (actual Unistrut part) like at the top where the beam is doubled, and the one that is wider to allow a bar to mount across (Home Depot kind) like where the vertical struts are doubled, notice the gap, to allow proper mounting of the horizontal struts.
Strut was obtained from home depot, price break when you buy qty of 10. Nuts, bolts, expensive, perhaps buy in bulk? Actual Unistrut parts expensive online, I used aftermarket strut hinges, but they were so sloppy the pins were starting to slide through, I had to drive out the pins and replace them with clevis pins. Afterwards I found an electrical supply store in Brooklyn (I believe) with a web page, prices were reasonable. Think I got the side by side two tab plates from them, and a hinge or two for comparison.
Edit: I originally used medium duty stem casters (think that is what the original designer used) from harbor freight, but could never get them tight enough, this design is heavy, and the wheels were wobbly. The second person in that thread used “U” bolts over the strut to use four hole heavy duty casters. I purchased heavy duty casters and filed out two of the holes and rotated the plate to use 3/8 strut bolts and a channel nut. Much better.