Here is how I did it:
Measure the slope, using a square angle (tool), relative to the floor boards. Take two height values a known distance apart (E.G. a foot apart). With these three measures the angle is easy to reproduce on a piece of paper. Then copy the angle from the paper to a sliding bevel (tool) and transfer it on to the wood. Before you start cutting, compare the sliding bevel one more time with the aft slope of the CB case, just to be sure.
If you don’t have these carpenter tools available, you can also cut the paper with a pair of sizers and use the paper as a stencil on the wood, similar to Algol’s method.