I use 3/4" laminated MDF board and generally I drill will a 3/8" drill bit (or whatever would be snug for the probe you use) a slot that goes all the way from the rear to the middle of the flex watt. Then I resin the probe in place. It is a pain if you ever have to replace probes, but it is not going anywhere! Also it puts the probe under the flex watt so no chance of a tub hitting it or anything else for that matter.