Hi Roger,
The "Commode mount" is illustrated below:
The basis is two 3/4" "water closet" PVC plastic flanges, which hold the toilet to the floor. The center post can be 3 or 4" PVC Schedule 80 pipe. I have made at least half a dozen of these, and the original was on a trawler almost 20 years ago and is still in service. The design came about as a friend was visiting and we went to the local scrap metal yard to find materials--nothing suitable in Aluminum, so we tried the plastic.
I bolt it direct to the cabin roof. All bolts are machine screw flatheads, so that the surface is smooth. The upper mount the machine screws heads are counter sunk into the Starboard, and Nylox nut on the bottom. You can prepare a flat surface, on the C Dory roof, but the PVC is flexible enough to accommodate the C Dory camber. I used 5/16" bolts and fender washers under inside the cabin on the lower mount, as I recollect. The top platform is either a thick cutting board of high density polyethylene from Walmart, or Starboard 1/2". The more recent ones have used the Starboard.
The height can vary, but the one shown is 12" for the radar beam to clear the air conditioner and loud hailer (also a GPS antenna on the Tom Cat). I probably would not go more than 18". You can put a "down slope" to the beam, if you wish--I have used stacked washers or Starboard wedges to give a 3 to 4* down slope to account for the bow rise when gong faster. However most of the time I am going slowly when under radar, and don't need a wedge.
The extra grooves in the base can be filled with 3m 4000 or the similar Loctite Marine PL adhesive / sealant. Paint is any of a number of "plastic" spray paint. I have used Rustoleum all purpose or Krylon Fusion. Both will give a good finish.