My brother-in-law uses a lot of MDF, especially melamine-faced, in his cabinet business, so I am often able to get scraps. At one point he had a stack of a couple hundred pieces of 3/16" melamine at about 18"x48" each after a massive commercial cabinet job. I won't need to go get more any time soon, as I took a couple dozen pieces home.
The beauty of this is that it is the same thickness as the hundreds of pounds of scrap A36 flat stock I have from a shop where I worked. I also have 1/2", 1", 1.5", and 2" square tubing scraps. It's very easy to make up those sizes from 1/2" and 3/4" plywood and 3/8", 1/2", 3/4", and 1" MDF scraps. When I have some scraps of those that are too small to be very useful for other things I rip them into strips that match the steel scraps I have. Then when I need it to mock up a project I don't have to set up the table saw, I just quickly cut the previously prepared "stock" to length at the miter saw.
Here's my first draft of my 2"x72" belt grinder:
(The dowel section represents the gas shock I'm using for belt tension, at full extension.)
Some of what is solid here actually represents tubing. Note how well they correspond:
I try to do this to actual dimensions I will be using in the final build so I can just reference from this for all my cuts and layout later. I even have one corner of the base plate here designated as "Reference", with every dimension from that corner a nice easily repeatable fraction.
Mind you this is the actual build currently in progress:
I learned enough from the mockup to change my mind about most of the design. Then I was confident enough to proceed without doing another mockup.
I don't know if this will help anyone, but hopefully it will get some ideas started/planted for a few others.