I laid out the rack on my garage floor. It is nice and flat, so I will use the floor to keep the rack in line as I weld it. I don't have a big welding table...
Here are some pieces getting fitted:
20171115_140450 by
sunpilot1, on Flickr
here is a close up of the rear corner
20171115_140459 by
sunpilot1, on Flickr
Here is the front corner
20171115_140506 by
sunpilot1, on Flickr
20171115_140442 by
sunpilot1, on Flickr
I am using 45° corners, and if you do also, remember to make each cut at 22-1/2° so they equal a 45° angle.
Here is a pic of getting things lined up
20171115_143411 by
sunpilot1, on Flickr
20171115_143424 by
sunpilot1, on Flickr
After I got the frame squared up I did a tack weld on each joint, then went back and did a more solid weld. The rack came out nice and square and wasn't racked or warped at all...
I set the rack on some jack stands and did the finish joint welds and I ground the outside and top and bottom of the welds to give a smooth surface. I left the inside corners a little heavier to give the rack more rigidity at the corners. I also added the cross bracing at this time to strengthen the rack and keep it in alignment, especially when it is carrying the tent.
20171121_130726 by
sunpilot1, on Flickr
20171127_151915 by
sunpilot1, on Flickr
You can see the steel parts resting on the top of the rack, getting ready to build the cage. I bought some mesh to use as a floor...just sitting on the rack loosely for now and it helps hold the tools!
20171127_151854 by
sunpilot1, on Flickr
The bracket you see hanging down in the middle of the rack is where the thrid leg will attach. This leg is similar to the main support legs, but doesn't get bolted to the roof (that would have meant drilling holes in the roof, and taking the headliner down, etc. A lot of work! Instead, the middle leg will rest on nylon blocks placed in the roof channel, and give support for carrying the tent weight, so the rack frame doesn't sag.