It's extremely important not to cover the front of the device.
You can try using a ton of tape, but be very careful not to get near the window. Try making squares or strips of tape and put about 1 cm of it on the side of the aluminum walls of the Leap and then the rest on the HMD. Something like a 2cm wide by 8cm long strip is ideal. You may need to also tape the cable to the HMD or the Leap USB cable to the HMD USB cables to make sure the cable pressure from it doesn't pry the Leap off the surface. If the cable droops in front of the sensor it may interfere with tracking...
Make sure you don't use any USB hubs for the Leap (this will slow down the fps which is important for VR). Make sure the Leap is not mounted in a crooked way and is centered on the HMD and that it's looking straight out (this isn't extremely sensitive but it will hurt your experience if it is off by a lot).
We did this a lot at first but over time it can obviously gunk things up (from all the tape) and removing the Leap and then putting it back on the HMD is a huge hassle.
That said... there's a lot of things that can go wrong in this process... and this is why we decided to make a standardized mount for everyone... This is probably the only way we can ensure a good experience for a large number of people...
