It's a pretty simple project.
1. Take a 2x4 or similar wood and trim it so that it's about 1" shorter that the intended bench width.
2. Measure down from the gunwale and make a mark where you want the top of the bench to lie. Make the same mark on the other side of the boat.
3. Since the topsides aren't perfectly vertical, you need to get the angle off vertical. Use a carpenter's sliding bevel (avail at most home centers) to record the angle.
4. Rip the 2/4 block using angle recorded with the carpenter's bevel. You'll end up with a 2x4 that has one side that isn't parallel to the other side.
5. Align the ripped 2x4 block with the mark on the dinghy where you want the seat, BUT LOWER IT by the thickness of the bench. If the bench is 1" thick, lower the 2/4 by 1". Mark the 2x4 position on both sides of the boat. Note that the ripped side of the 2x4 should be against the outside of the boat. You'll have to reverse the cut for the other side. The idea is that the bench lies across the top of each 2x4, so the tops ahould be on the same plane and the bench lies flat.
6. Mark the outline of each 2x4 on the dinghy and rough up the fiberglass inside the marking.
7. Have an assistant hold the 2x4 in place while you drill from the outside of the dinghy through the hull into the 2x4. Secure with countersunk SS screws. Back the screws out until the 2x4s come off, but leave the screws in the topside holes (it'll make assembly easier in Step 9).
8. Mix a little epoxy and a little thickener (so that it doesn't run). Tape off the area outside where the 2x4s will be glued. Use lots of newspaper.
9. Apply the epoxy to the 2x4 (NOT the hull) and secure with the screws from Step 7. Wipe the excess epoxy with a paper towel & Laquer Thinner (NOT acetone, it'll soften the dinghy's fiberglass)
10. Cut a bench from plywood, solid wood or whatever but it should be at least 1" thick. Taper the ends to match the fore-aft curve of the hull. You may also have to taper the cut to match the flare (top-bottom) of the hull.
11. Attach a 1"x2" strip along the center underside of the seat (attach the 1" side to the seat). This will stiffen the seat so that it doesn't bow when you sit on it.
12. After the epoxy in Step 9 hardens, attach the bench to the 2x4 blocks with 2 #12 SS countersunk screws on each side . Don't glue the bench, the screws are more than adequate to hold the bench in place and they allow bench removal if necessary.
I'd upload a photo example, but the file XFER keeps failing, so you can see an example from a dinghy that I built at the following address: