i got display rack (can be had for like $10-$20 a sheet online)
they are 1/4" bars in a 2"x2" grid. cut and welded how i wanted. i used a full sheet on the bottom which gave me my dimensions. then i useda second rack to make the sides. I got 1.5"x2" box and welded it length wise along the bottom (3 different box tubing segments) then i took some 2" angle and welded it on for light mounts and bike mounts.
All of that is welded to 2 thule load bars.
Supports over 600 lbs while driving i have had 1000 lbs in with no issues(and then took it down becuase it was to top heavy).
I use it for just about everything