I originally planned to make the frame from 20mm box section steel, but before welding everything together I had the good sense to check the rigidity of a length the same as that of the trike. It was nowhere near strong enough so I changed the design to use 1 1/4" (28.6mm) x 2mm tube. 

I cadded it up in Fusion360:

It's a classic tapered frame design with 3 crossbars in 20mm box section to strengthen it. To fit everything together my plan was to notch the box section.

I'd bought a JD2 tube notcher for this (it can do round or square material) and the position of the notches is going to be pretty important! But the frame also has a taper:

This means I need to get both the position of the notch and it's angle right. To accomplish this I created some guides in the CAD model that I could use to align the hole cutter on the notcher to the correct position:

By clamping these at the end of the box section, I could then move the material to get the notcher's hole cutter in the correct position and at the right angle:

This worked really well. So I started welding it together.

The tracks are mounted to the frame on flange bearings, two per track. I created mounting plates to attach to the 20mm box section cross members, with some reinforcement plates to be welded on the back.

And these were welded into position at the rear of the frame.

A 20mm steel rod sits between the bearings, and an arm bent at 90 degrees welded to the track unit frame allows it to move up and down. A shock absorber will be fitted later.

I used the JD2 bender to create these.

I plasma cut some brackets to mount the shock absorbers and overall, it turned out pretty well. Next: Steering.

There's a video  blog of the frame being built