I've had quite a lot of trouble getting a pitot cover which fits. The Musketeer has a really narrow tube, and I couldn't source any removable cover which would fit.

The Western Airmotive guys ordered me a permanent-style aluminium model. The first one that came was too small. The replacement was initially too big, but a bit of plastic tubing between the pitot clamp and the tube did the trick.

It's a clever-looking design - it automatically opens when I get enough airspeed. It would add a bit of wind resistance, but Squibby is such a bus anyway that it's not going to make much difference. And I don't have to remember to remove it before flight.