I had a couple of commercial L shaped back cushions from another boat that didn't work well in my usual seating position on Monita because the winches and cleats on the cockpit coamings got in the way. I thought about cutting a notch in the cushions but they are about 2" thick and have a smooth colored finish. Cutting in to them would leave the thick raw white insides exposed. Not wanting to throw the expensive cushions away I decided to cut them up anyway and try painting the cut edges. Some moron who shall remain nameless totally botched the first one by cutting a huge notch in the wrong end. Senior moment!! The other one turned out ok, and the green Rustoleum paint from a dinghy project turned out to be an almost perfect match.

Then I decided to try making a similar cushion from scratch. I laminated two layers of 1/2" closed cell foam (camp sleeping mats from Walmart) with contact cement to get a 1" thickness, and glued two pieces together in an L shape. I had some Sunbrella fabric left over from my sail cover project so I used contact cement to glue it to the cushion as a cover. The folds on the back side don't look too neat but they are not visible. The exposed side looks ok.

I glued a small sleeve of Sunbrella to the inside of the L on each cushion and threaded a length of 1/8" shock cord through. I tied the ends of the shock cord together to form a loop to hang over the winch or cleat on the cockpit coaming. This keeps the cushion in place when tacking.

Both cushions work well but I find I prefer the thinner home made one.