Jeez, that was so easy. I was differencing the tubes (to make a single hollow tube) and then unioning the result to the model, then deleting the surfaces that intersected the tube, which caused all kinds of problems, and I still couldn't get an opening in the inclined plane. Anyway, unioning the outer tube with the model and and then differencing the inner tube accomplished exactly what I wanted. I'm still learning to think like MoI, I guess. Thanks a lot.