Tom, it is likely to take more than a grommet to adequately seal the point where the probe enters the drop checker. That connection has to be air tight, or the device doesn't work. Something more like the sealing nuts used for wires going into a sealed device would be best. And, given the relatively short life of the probes, you can't just use silicone sealant to seal it. I think the best design would be made from two sizes of acrylic pipe - the inner one about twice the diameter of the probe, the outer one about twice the diameter of the inner one, and the inner one with a bottom piece that extends to attach the two together. At the top would be a solid piece of acrylic with a liquid tight pass thru fitting from Heyco, Handles, Thumb Screws Captive Panel Screws, Stand offs, Handles, Connector Hardware, Lyn-Tron, Inc. - Spacers, Thumb Screws, Captive Panel Screws, Standoffs, Handles, Connector Hardware - Bolt Products Inc., Fasteners, Cable Ties, Distributor Standar, which fits the probe diameter.