I use crankbaits down to 50 feet on Lake Washington for smallmouth all winter long, not by modifying the bait, but attaching the crank to a three way rig. The idea is as old as the hills, but most guys have forgotten about it since most bass tourniments won't allow slow trolling. But for the guy who enjoys fishing just for himself and likes to experiment, it works great!
Basically, the line from the rod is tied to a three way swivel. One of the other swivel tie-ons is used to add a weight on a 18-24 inch leader. (Use a line a couple of pounds lighter than the main line so that if the weight snags you only loose the weight.) The other line tied on the swivel goes to the crank - also a 18-24 inch leader. My favorite and most successful crank is a shallow runner - the Rapala jointed minnow. But I've caught dozens of bass on numerous other cranks in the tackle box. (It's an absolutely deadly rig for mega perch in the fall, using a tiny crank.)
Find a flat, turn your electric trolling motor on 4-5 (I find the faster speeds don't give the fish too much time to think about it, and you will get reaction strikes - though everyonce in a while the slower the better, even dragging the bait works). Use scent! Any bass fisherman who is patient enough to drag dropshot rigs can also drag this crank rig. Don't be afraid to use large weights to get down to those winter depths - I've used as large as 3 oz, to cover flats in 50 feet of water during the winter. (I try to keep the weight 6 inches to a foot above the bottom. Believe me - my underwater camera has shown me that any smallmouth in the area will come tp check it out.) And remember, if it's following your bait, dropping the rig back occassionally will cause the crank to suddenly appear in it's face, and often get a reaction bite.
But this technique also works great in summer when the larger fish move back out after the spawn into the summer and fall in deeper lakes. Believe me, it works just great in 20-30 feet of water. My local bass club doesn't have the "no trolling" clause and I've used it very effectively on those hot summer days when everything in the shallows closes down. The larger fish in deeper water doesn't seem to be as affected by fronts coming through.
Believe me - it works! It isn't as exciting for the type A fisherman who needs to be casting every 5 seconds, but it is one more tool to add to your arsenal. The bait is always in the water, at the right depth, and nothing covers a flat quicker! I could go on and on about this technique, but the fact is - it's easier to attach the crank of your choice to a deep three way rig than modify cranks to go deeper. Well, that's one opinion anyway.