I believe it was last summer we had a discussion on the use of a float on a fishfinder rig. Here's my opinion. If you want to use the fishfinder rig you have to carefully match the weight of your bait to the bouyancy of the float. If the float is too bouyant for the bait, your bait will float up near the surface...usually not a good thing. So, IMO if you really want to use the float, lose the fishfinder rig.
If you really want to use both you can either trim your bait to match the float or the float to match the bait. Here's one thing I've used with success but again it takes trimming. I use a cork from a wine bottle and trim it. I then slide in on the hook and bury the eye in the cork to secure it. Cheap, but effective.
If you're under-age, have your father buy a case of good Chardonnay, I'll drink it for you and provide you with all the corks free of charge. I think the fish prefer white wine corks. :)