I made two of those floating boxes, one for the hot tub and one for the pool skimmer. One I drilled out a little on the large size, the other I drilled a nearly exact fit hole for the probe, sealed both up with silicone gasket maker. The larger-holed box ended up leaking through the sealant and killed the sensor. I think the probe could wiggle around in the hole a little bit and eventually wore a hole in the sealant.
The exact-fit box is still going strong