Here's a quick way to experiment with stall strips that you might want to consider.
Take some thin cardboard, such as the material in a postcard or a manila file folder. Cut a strip about 3/4 inch wide and with a length of about 1/3 of the span of one side of the wing. Fold the strip lengthwise with a sharp crease. Glue a small wedge of balsa (or foam) at each end of the folded strip to hold an fold-angle of about 45 degrees. Cut out the base of the wedge so it fits around the leading edge of the wing (precision isn't important) and allows the edges of the cardboard to rest against the top and bottom of the wing. Position the folded strip with one end against the side of the fuselage and with the folded edge aligned with an imaginary line that connects the front and rear of the airfoil-shape. Tape the folded strip to the wing ("Scotch" tape works fine). The sharp edge of the folded strip will protrude out beyond leading edge of the original airfoil.
Repeat with the second-side of the wing, with a structure that duplicates (as close as you can) the structure on the first-side of the wing.
Now you can fly the plane to see if the strips alleviate the tip stalling. If yes, you can replace the cardboard assembly with more permanent structure. If no, you can easily remove the experimental strips.