JAGS,
I made a helm seat for my boat a few years ago. I made a teak ladder back and teak armrests with a cushioned seat bottom and with a removable cushion for the ladder back. Sorry no pics right now.
Lining up the hardware was the hardest part. I found some arm and back brackets but they were not up to the quality of the seat I had in mind. I finally found brackets via a company called Estran whose web site appears to now be gone. Even then there were issues, the bracket quality was great but came in powder coated, I ended up stripping them, polishing and then taking them and had a bright annodize finish applied ($50).
I never did find real high quality hardware available retail to match what I had seen I some of the high end seats.
The ladder back was made of teak and the horizontal and vertical pieces were grooved to interlock, glued and screwed. The vertical pieces were thicker to take the load and the horizontal strips were thin maybe 1/4 inch. The teak arm rests were actually two parts glued together to get the thickness I wanted. This allowed a recess in the bottom to except the flat end plate of the arm bracket to hide that and enough depth to the wood to take 4 screws in each of those plates.
The bottom was just treated plywood to make it waterproof. I didn't want to make it too tough so that the upholstery shop could not staple to it. The bottom piece was layed out and recessed holes put in to take t-nuts for all the slider hardware bolts the back rest brackets and side arm brackets bolts.
I had a 4 inch cushion installed and the front edge was made seamless and rounded so when running the boat standing it was comfortable to lean your butt against.
Tips:
Check the width of the seat when installed centerline with the wheel hub. In my case the boat was an express with the helm to stbd, and I had to watch the width of the seat to allow the armrests to clear the side when rotating it 180 degrees (beer drinking mode)
Check how much throw you get on the slider you choose before making holes in the seat bottom, normally you'd probably center this fore/aft but doesn't have to be.
If your installing on a bridge your going to want the bottom of the seat to look good too when looking up from the cockpit.
Because of the cost of the teak and not wanting it scratched I supplied the upholstery shop with a plywood template of the ladder back for them to make a removable cushion. (turned out to be real good idea)
I made a template down the basement and made the seat fit me for the seat height, width and even the fore/aft and height placement of the armrests.
It of course wasn't as cheap as you'd hope with the hardware, teak and upholstery but I can't wait to do it again as soon as I can get the next boat.
Brian