The GNAT's seat is made from fabric, avoiding the 'sweat bucket' effect of certain other designs (if you don't know which I mean, then perhaps you don't know enough to make a purchasing decision). You may be able to get a readymade seat with the kit, but here we assume you are sewing it yourself (else have persuaded your wife/girlfriend/mother/daughter to do it) and give some basic guidelines.
There is quite a lot of wear on the seat, so you should choose a thick material, with a high cotton or linen content, which will breathe, and won't stretch. An upholstery shop is a good place to buy the material; if you go to a dressmaking shop and ask for heavy fabric they might sell you something that would make a thick ladies' blouse, but a flimsy trike seat.
If you are like me, you will just say you're upholstering a seat when the shop assistant asks you what the fabric is for - true enough: a GNAT is a seat on wheels, after all.
You should start with a piece of material somewhat bigger than the dimensions shown on the right, to allow for the seams. The material is folded back at the bottom, to create a pocket into which a piece of wood is sewn. The wood should be roughly 12" long, half an inch thick, and 2" wide, so you need about a yard of material for the main piece.
The fabric has an envelope at the back of the top, the curved line in the picture, to contain the top of the metal frame. This needs another piece of material. It is a good idea to reinforce the main piece in the middle, as shown on the right. Not only does this make it less likely to stretch where the sideways tension is greatest, but with the printed material shown, it presents a tidier appearance.The metal frame of the seat back should fit tightly into this envelope, especially in the stress region where my hand is in the picture on the left. If you find the fabric is a loose fit, you should turn it inside out and sew the seams slightly closer together.
The middle height of the back piece should be no more than ten inches, hence the curved line in the middle of the diagram, else it is dificult to get at the balljoint at the back of the seat. You can see the bottom of this joint in the picture on the left. This joint needs to be accessible so you can, with one hand, squeeze it apart and fold the seat forward - very useful if you want to store the trike sideways.
Unlike most recuments, with the GNAT you choose exactly the seat angle that suits you. (You have two adjustments to the GNAT seat: the angle; and the length of the straps pulling on the front of the seat. You also set the boom to the required length) You are unlikely to want your seat as laid back as the one shown above, however. This was Paul Lowing's GNAT on which he entered the Brighton IHPVA world championships. (Paul - seen here preparing the fairing in my workshop with his wife Carolyn - was involved in a collision at the start of his first race; my first and probably last attempt to sponsor a racing team)
If you don't make the sides of the envelope deep enough, you find that the bottom of the seat rides up the back, as in the picture (right). This looks odd, but doesn't affect comfort if you have lengthwise tension on the seat. It is better to stick to the above dimensions, however.
I like to have a pocket on the back of the seat, for carrying small items like a spare tube (you need only one size on the GNAT). This requires a third layer of material at the top of the seat. I usually use fluorescent mesh, with thick elastic along the top (left). You need to put some tucks in.
You should round off the top of the wood stiffening the front of the seat, into something like the shape of an aircraft wing. Make sure you sew it in the right way up. I have a large amount of red nylon webbing (left over from the days I used to make panniers - with the trade name Bikeasy). Two pieces of red web running down the entire length of the seat are shown in the green diagram. I recommend you do it this way, unless you are using very substantial material. However you do it, you need somehow to attach the front of the seat to the front arms of the wishbones, using pedal toe-straps. I usually use two thicknesses of web where the toe-straps join, as it wears quickly here. The centres of the web at this end should be about 8.5" apart at this end.