Fins made from the top of a bottle
These fins are made from a 1.5 litre or 2 litre bottle that you will have to butcher (I mean form) into the correct shape. They are not the prettiest of fins or even the best of fins, I suppose that you could even argue that they are not fins, but they are easy to make, will survive numerous launches and landings and you don't need any skills, materials or machinery that you haven't already got.
First, cut off the neck of the bottle so that the resulting hole will only just fit over the screw thread of the rocket bottle - ideally it should be so tight that you hace to screw it on - the acceleration ivolved means that anything that you use to attach anything a rocket will have to support many times the weight of whatever it is that you are putting on. Cut off the bottom of the bottle so that you have 100mm - 200mm of bottle left (make it longer for a larger bottle).
Use a hot soldering iron (or hot nail in a pair of pliers) to melt four to six holes through the side of the bottle, evenly spaced, approxiately where the sides straightne out from the neck. Cut from these holes, down the bottle to the bottom of the bottle to for equaly sized flaps.
Then, fold along each of these flaps so that they form a V in cross-section and look more like fins.
As it is impossible to get the nozzle in perfectly straight (it certainly won't be if the rocket lands on it), you can make the rocket rotate during the flight by turning the corners alternately in and out (as in the diagram on the left) so that it will spiral during the flight therefore cancelling out the effects of all but the most ill-aligned nozzles.
If there is not enough spin, the rocket will loop - too much spin and the rocket will form a vortex inside which will let the air out before all of the propellant is used up. You will also have to recalibrate the drag on the computer model if you are going to use it to predict the height of a flight although, by not altering the nozzle or the pressure, the optimum mass of propellant should not be significantly different.