I have a desk made out of the trusty 3/4" threaded stainless steel rods and plywood (oak veneer). I figured that similar construction methods could be used to create a pretty nice rack.
I had 12' of 3/4" stainless threaded rod and several grade 8 matching 3/4" nuts to be used. I also had some 3/4" MDF from another project.
So, here's the before picture, 48"x36" to be cut to 4 shelves 24"x18":
Next measure in 1-3/8" from each edge and mark. If you have a set of spanners or scribe this makes things very fast and drill 3/4" holes for the threaded rod. You can go a little bigger, but I don't recommend anything over 13/16" as too big a hole makes the whole thing wobbly and tough to put together. I'd rather have the rods snug in the holes.