I made this model because the existing ones usually had issues when reversing.
There are two major changes in my design:
To use the skis you have to remove the front drive shaft.
If you have around 10mm of thread outside the hex on the front axle you can put the skis over the hexes (depending on the crawler you might have to be extra careful to tighten enough to keep the hex in place so that you do not lose the pins but allow the skis to pivot at the same time)
Another options is to remove the hex, but you might need to use a drill to increase the hole in the ski.
A test video with the skis can be seen here (please ignore the wheels, they are a failed experiment, there is no grip). For reference, the crawler in the video is Element RC builders kit 2 :
Printing instructions:
The author marked this model as their own original creation.