Borna:

Nothing you can do to ease the tip-stall will be pretty. A stall strip can be as simple as a popsicle stick, cut in half lengthwise and glued to the front of the leading edge sticking straight forward. It should have the inboard end at the side of the fuselage.

The problem with stall strips is that you need to experiment to find the right size to do what you want to do. And while you are experimenting, you are probably crashing.

Call Horizon Hobbies and order a set of leading edge droops from their P-51 or F-22 trainers, and make them fit your airplane. They will be ugly, but that's the only way you have any real chance of eliminating the tip-stall on the first try. The other benefits of the droops are that they add to the wing area (reduced wing loading) and they make it stall later, not sooner like the stall strips.

Have you considered donating the Chipmunk to a school or charity as a static model and writing it off on your taxes?

Jim