The trad way is to make up your own mitre block on site, to fit size
of moulding. Just 2 bits of say 2x2 inch nailed together to make an
L. You then cut it at whatever angle you need - having marked it up
very carefully with sliding bevel and square.
If you are canting up a flat moulding to make a cornice you stick a
couple of nails in the bottom piece of the block to hold the piece at
the required angle (45 deg or otherwise as long as you are consistent)
while you cut it.
Most of the bought gizmo's are more trouble than they are worth and
won't do everything you want anyway.

cheers

Jacob