I have assembled the slat seats that came with my 16.5 but haven't finished them yet - for now I have two Creature Comfort seats in the boat. For slat spacing I followed the drawing that came with the kit to get the slats closest to the intersection of the seat and the back in the position as shown on the drawing. Also the top slats of the seat backs and the forwardmost slats of the seat bases. The drawings are to scale so this is straightforward. For the rest of the slats I positioned them all without nailing them down and adjusted the spacing until it looked right, using my calibrated eyeball. Then marked their positions with a pencil and nailed and epoxied them in place, one at a time. Marking with a pencil is important as the slats will move out of place while you are nailing. Sitting on the assembled seat on the garage floor they feel like they will work fine. I plan to epoxy the seats with one or two coats and then varnish. The Creature Comfort seats are OK but I think the slat seats might offer more back support, we'll see. A time crunch led to the purchase of the CC seats, if the slat seats work better I may have two CC seats for sale soon. 

  @JohninSD - Thanks.  More or less what I've settled on.  Bound the 4 rail pieces together and marked lines at 1" spacing with a square so they'd be true and evenly spaced, not quite trusting my poorly calibrated and aging eyes.  Made a little jig to drill the 88 pilot holes in the slats efficiently and true.

Not sure what I think of the flat head machine screw for the pivot between back and seat - doesn't seem right.  So, off to the Home Improvement Center for a round or pan head stainless alternative.  Likewise the flat head wood screws for attaching cross bars - countersinking in plywood that thin seems a little dicey.

You got a drawing of the seats in your kit?  I got nothing...  just the pieces.  

 The seats look and work great.  I followed more or less the same process (though mine came with full size plans). 

A couple of things to note from my experience - my kit did not have any sort of adjustable buckle to connect the straps from the top of the seat back to the shear clamps. I ended up using plastic cam locks on the straps, which don't seem to be tight enough when a person leans back on the seat, something more robust is needed.  secondly, the drawings show screws that hold the straps to the top of the seat back being driven into the sides of the seat back - which effectively goes into the end-grain of the cross-bar and resulted in the screw popping out on the first paddle... I had to epoxy it back in.

I used velcro to secure the seats to the kayak.

   I used spacers cut from scrap timber to ensure the slats were evenly spaced on mine. That all worked well. I am not convinced that they are the best solution for seats, although I like the aesthetic. I made up one inch foam pads for the seats and backs which does improve the comfort somewhat, and used velcro to stick them on. My biggest gripe is that I feel the backs are not high enough to give my ageing back the support I require. I might try building taller backs for them as an experiment . Was out for an hour today in the Solway Firth in a stiff breeze and felt that I didn't have enough support - my stomach muscles were complaining volubly by the time I made land.

   Just a quick update on the seats. We now use the slatted seats and the Creature Comfort seats - just put the CC seats on top of the slatted seats. This adds a bit of height for better paddle clearance and adds a bit of comfort. 

I modified the slat seat installation by adding two "Footman Loops" to each seat one at the upper rear corner of each seat back. Then I use a piece of 1/4" braided nylon line to go from a brass padeye screwed to the underside of a carlin, through the footman loops and around to a jam cleat screwed to the underside of the opposite carlin. I also glued heavy duty weatherstripping to the underside of each seat side and find that this provides enough friction to keep the seats in place while we paddle. 

I built this boat because I like the versatility of its design, i.e. it can be paddled, sailed, rowed or powered, can accomodate one person or two, etc.  One problem I had when paddling it alone was that sitting in the rear seat caused the bow to come up too high. But with the seat adjustment system that I have now I can use the front seat when paddling alone. I put an extra long piece of nylon rope in the front position and find that I can put the front seat back far enough to balance the boat nicely while still reaching the forward footrests adjusted to their rearmost position. I do have extra long legs so this may not work for everyone. 

If I need a winter project maybe I'll take some time and figure out how to post a picture here to illustrate this more clearly - but right now I'm going paddling (weather forecast is for high 70's today at the beach (Nov. 13 2021) in sunny San Diego.