I had need to make some prismoidal demos for a physics class I was teaching to my senior employees a couple of years ago. To be honest, I wasted several parts prior to getting what I was in need of. If your roof is to be the shape of a prismoid (frustum of a pyramid), the best way I came-up with was to use my sliding compound miter saw, which is a Makita LS1013. I built-up an angled support to be bolted to the saw's fence.
Divide 360 (degrees) by the number of hip cuts that are needed (i.e. 8 if it's 4- sided or 10 if it's 5- sided), which gives you 45 (degrees). Adjust miter angle only, because your bevel part of the cut is taken care of with the workpiece being supported by your angled support. It's a tricky cut, that needs precise alignment - but after making a couple of cuts you'll realize it's a smart and safe way to make the cuts. Routers are wonderful, but 68 degrees is not an angle best handled with a router. OPG3