Mountain Man,
From your description I suspect you are not looking for a Hydrofoil as described by Polliwog that raises your whole boat out of the water. I assume you just want to add some version of a stabilizing plate or wing to the cavitation plate. Such as products made by Dolphin.
I had a 115 hp without, then with a Happy Troller plate. Although this product is mainly for slowing a large engine down during trolling, it also has the advantages of a stabilizing plate when the troller is in the up position. I can tell you, that the plate which sticks out about a foot longer than the cavitation plate does make a difference at low speed. Without the plate you need to go twice the speed for minimum plane speed. This also allow quicker planning.
Of course some would say not to put a plate on your engine. Just drop down in prop pitch and it can fix the problem. Sometimes your lucky and this change does not change your top end. You could also change from a three blade to 4 blade in the same pitch.
The plate on the lower unit could be your most inexpensive fix though. Remember is a boat is set up right you should never need to add anything to the lower unit.
Good Luck,
BATMAN