I think making a wedge camper similar to how they have over on Tacomaworld might be the best solution: How to: DIY Wedge Camper - https://www.tacomaworld.com/threads/how-to-diy-wedge-camper.604686/ You could get the exact dimensions you want/need without having to piecemeal something. There are designs in there for other than just the full canopy setup. You could just build out the top section and call it good.