A very pertinent question!
If the part that is actually in the water doesn't look substantially better there is a cheap(!) way to give it a bit of the missing profile.
- Extend the CB.
- Apply a goodly amount of 3M High Strength Marine Filler to the edge (it's fiber reinforced so unlike other "fillers" it can support itself even when its "proud").
- Use wax paper or a stiff sheet of plastic to "mold" the filler into a half-circle shape.
- Sand to taste
You can use the same technique on the trailing edge to get a nice wedge shape (that ends in an 1/8" sharp cutoff, slightly angled, that is, not 90 degrees). The use of wax paper/plastic sheet is really the secret to adding a wedge (you will add .25 to .5" and that should still be class legal and / or fit in the CB trunk)
Now, the third thing you can do:
- apply an epoxy-based fairing compound to the front 1/2 of the CB. (West System with micro balloons or SystemThree QuickFair).
- Apply it with a notched spreader and let it cure.
- Sand into a continuous, fair curve with a long sanding board
- Fill any grooves with more fairing compound
- Sand to taste
With these three simple steps you can improve the foil shape of your CB with limited effort/expense. The goal is to get the leading edge to be round, the first half of the CB to be wider so that the max width is about at the 1/3 mark, and the remainder to narrow and taper into a wedge. You won't be able to get the full NACA foil shape, but it will be an improvement over the stock shape. I did this a few years ago and when before the CB was "thrumming" at higher speeds it is now quiet. (The stock shape is too thick in the aft 1/2 of the profile, which limits how closely you can approximate the foil shape, but there should be room in your CB trunk to make the leading half of the board a bit wider, which helps get you closet to an ideal shape).
When done, paint. You can use deck paint if you dry-sail your boat. If you plan to take it overnight, or have it on a mooring, use stuff rated for underwater use.
Then, save up for a real CB or learn how to build one from scratch, to get even better performance.