Get a non-slip carpet pad, and just cut it to a convenient size and place it under the lettes. I do this for mine (I use PVC, they also slip like hell). It's super sticky, durable, and cheap. Get the smallest one that works, but make sure it will cover the span you need.

Any sort of sticky tape will lose its stick fairly quickly, and it would be hard to apply enough to the bottom of those anyway. The pad is prob the best bet.