For some things, all you need is some industrial strength clear tape, and possibly some thick PVC film. This will enable you to cover various aerodynamically ugly depressions or put boattails on appendages.
1. Clean the area well beforehand, as well as your hands, with methylated spirits and a clean rag. This will enable the tape to stick, and you not to leave grubby marks on the tape.
2. Cut out the clear PVC with box cutters (larger items, for smaller things tape can be sufficient. Fit it to the area you want, making sure that if you are making a boattail that it taper than no more than about 15 degrees away from the horizontal or vertical in the direction of the car's center.
3. Cut tape and apply as necessary. Try and hermetically seal the areas you are covering, this prevents dust/bugs/water getting in there, adverse pressure gradients, etc.
Here are the results on my car. All I want to do now is fix the windscreen wipers.
