Hi DONI. There are two on eBay and I have tried both! The one with four legs that hold the tracker is crap as one leg broke rendering it useless. The better one is the cage type which I think I got from a guy in Israel approx £12 and holds the gps unit snugly. They are made from a 3D plastic and are not that strong?
Sorry just looked on eBay and forgot to mention the ones that fit between the legs DO NOT fit the rf v16 even though it says it does ( been there done that) and do not fit the p3p. If you look they are on a P4. One holder is cheaper £8 and the one I bought has gone up to approx £15. Also I have never had any interference from it but it is a twat to set up.