Apart from the angle brackets, you also need a chassis mounting for the central seat brace and rear suspension web. I did not include it in an earlier section as it is probably easier to make one when you can see where to put it and what it does. The design here need not be slavishly copied: you may find it easier to use a flat section (with the corners rounded off) rather than round to anchor the web. However you do it, you should mount this fixture just inside the chain aperture, and as low down as possible on the chassis.
The GNAT's wheels are around 20" diameter. I suggest you the popular 406mm tyre/rim, which has emerged as the preferred size- 406mm is the diameter of the bead that holds the tyre to the rim. It has become harder lately to get the 500A tyres I use on my own trike, which make a slightly larger wheel. The boom bracket plan here gives a slightly higher pedal height (relative to the chassis) than the pictures of my own trike on this site. The height difference is around one inch, obtained by offsetting the lower boom slot forward by about 8mm, as shown on the right where my hand is holding the higher bracket.
Another way to alter the height of the boom, if you need to, would be to offset the boom clamp holes (left). There is plenty of thickness in the carbon mouldings at this point, so drilling extra holes would not weaken them excessively.
It is the custom for competition trikes to have high pedals. This might reduce wind resistance when racing around tracks, but for general use I recommend having them low. This makes for easier hill climbing, as you don't have your feet so far in the air. And the position is more comfortable even on a level surface. Being able to touch your heels on the ground might be worrying at first, but when you get accustomed to it you have a useful reverse gear.
You can adjust the height of each wheel independently on the GNAT, so you can trim the suspension to adjust the pedal height. You will find it advantageous to have the left side of the trike slightly higher than the right (riding on the left side of the road) to keep yourself more upright, since roads are usually crowned - higher in the middle that the sides.
The GNAT uses one-inch-wide webbing for suspension. You can get this easily in nylon, polypropylene and probably other materials, and in a variety of colours. I usually use a thin red nylon webbing. The shot on the right shows how it is clamped, using 5mm screws in bosses brazed onto the wishbone ends. The light-alloy clamps are 1/2" x 3/16" with holes at 30mm centres.
Unless you are planning to race your GNAT, I suggest you fit the widest range of gears you can, with a triple front chainset and 8 rear sprockets. It is a good idea for the bottom part of the chain to run through plastic tubing, secured underneath the chassis with cable ties and/or sticky tape (left; see here for the reason for the tie-wraps in the picture).