Hollvan, you could use a full size topper and trim it to size. We have a Duvalay topper on our fixed bed double which has the bottom corner angled as is the way with lots of fixed beds. The topper was a little too wide so we took it out of the cover and trimmed it to the shape we needed with a sharp knife and put it back in the cover. We just tuck the surplus cover underneath, but the cover could be stitched to tidy up the surplus. If you already have the two singles, I guess you could trim one or both of the to the size you need thus eliminating any need to over lap.
Regards, Mike
Posted this before, not sure if it might help anyone reading this and easy to cut to size i would imagine: My wife really wanted a duvetlay, which is a memory foam base with a duvet attached to it, so semi sleeping bag / ordinary bed. However, they are a little expensive. So she made our own. She bought 2 double duvet covers, 2 single duvets and an ordinary double memory foam topper. We then cut the topper in half and placed it inside and down one side of the double duvet cover. Then she sewed a thread down the middle of the cover to hold it in place (we did leave an 8 inch gap at the top and bottom to make it easier to get it in and out after washing. then we put the single duvet in the other half, Bingo one home made duvetlay for £30 each. tried them this weekend and the kids / teenagers love them. Loads better than the sleeping bags.