All these tricks trying to re-invent the wheel I've never had trouble just using a coping saw holding the crown or base or what have you on a work table!
Jeffe, That jig is for coping walls at 135 degree angles (some people call them 45 degree inside corners but they're really 135 degrees and the miter is 67 1/2 degrees. I say all of this only to be sure we're on the same page). Most often, those pieces are pretty short - like the returns on a bay window or popout. With a tall workbench, there's usually room to make the cope cut, but the whole process is not fun. While David Collins says all inside corners should be coped, I prefer to pre-assemble those nasty 135 degree corners, mitering them and clamping them while the glue sets. And honestly, if the pieces are long, then I'll just assemble the miters right on the wall. Work isn't a bad word except when it's hard work. I try to avoid that. :) Gary
This is basic trim work. You don't need a special set up to cut copes. I've been coping crown for 15 years with just a normal jigsaw with the foot taken off. Long blade, careful to keep the blade follower bearing from touching. I've always just clamped the piece to the miter saw table. If you sight the crown in plane with the ceiling flat (the part that actually bears on the ceiling), you will see if you have backcut enough. Unnecessary after a few rooms, you will feel develop muscle memory. I will say it is best to learn with a manual coping saw so that you develop the feel for the cut geometry with the least amount of blade binding.