I was on the same quest as you, I found a few options that were cool but way out of my price range, so I built one.
I used all parts from the Robotics Auto Industry:
The beams are robot fence posts (2" square 1/8" thick)
The Gussets and flat stock was retrieved from scrap bins
The Window guards is Robotic fencing panels
The Chain running down the back is from a transfer conveyor that had a malfunction and sent a bunch of links into oblivion
And the tiedown brackets are lift anchors for a robot controller.
(all items were free, just had to pay for weld wire and wheels)
I didn't have anything to go off off so I took my time and carefully measured, cut all angles, tack welded and verified fitment, It took me about a year on and off working on it after work.
I sprayed it with a self etching primer and Raptor Liner. The finish is very durable, Its not 100% yet but i wanted to give it a trial run for noise, rattling, porr paint coverage, etc. I found a couple of spots that need cleaned up and a 2nd coat applied, I also want to add a couple more pieces of flat stock to it.
Im happy with the outcome, the top center of the rack sits level with the top center of the roof. I have 55 1/2" up top for a light bar / pods.
I added some opaque black plastic to the side screens for aesthetic purposes, I also added stainless hardware and some metal "trim" to the sides.
Hard to see the hardware in the last photo, but it has the plastic installed, 3rd photo is up close on the hardware before the plastic was installed.
(i know, w.t.f right? this is just the surface of my organization in life
)
I'm just lucky to have taken photos.