I made a couple for local trucks lately. It's not hard and takes about 4 hours.
The dimensions I used were 32" from leg to leg across the bottom, 34" from bottom to top (inverted V), 12" for the support post. I used a vertical tube welded to the legs at the mount point, then set it on pins that are welded vertically on a flat piece of plate. Bolt the plate to the bed floor so the carrier can be removed from the truck by pulling 3 pins.
The tire hold down plate gave me problems trying to cut out a good circle, I finally used a star tire Iron.
Cut the center out weld the butt ends on the sides of a short piece of tube and gusset between the bars.
8' 1.5"x.095 tube
3' 1/8x4" flat plate
24" of 3/4" all thread
1 3/4" coupling nut
1 3/4" washer
1 Star tire iron
3 3/8" safety pins
9" 3/4 rod
Total cost $42.
PM me your email if you want pics of the finished product.