Something no one seems to have mentioned yet is that stall strips extended forward from the leading edge are used to force that area of the wing to stall. They don't delay the stall. So you want to install them on the center portion of the wing and not the tips. The idea being that you force the center to stall earlier to save the tips.

Another option instead of stall strips would be to add turbulator strips to the wing TIPS to try to delay the stall. What you want are span wise strips made from two layers of electrical tape stripes that are slit from the tape to be about 3/32 to 1/8 wide. Stick these double layer strips to the upper surface about 1/2 inch behind the leading edge for the outer 1/3 of the wings.

If niether of these works then you'll need to figure out a way to warp in some washout in the tips.