Electrical Cable Covers

I am looking for a stock wire / cable entry cover. What we plan on doing is using a standard 16-2 cordset to feed a flat heating element we then need to cover that wire where it comes through the protective cover with a low profile plastic cover that we will fill with a sealant to make the heater moisture proofed. We have seen something like this that measures about1 3/4" x 1 1/8" (about .035 thick) it has 3 flat sides for gluing it down and an open area of about 3/4" x 1" x 3/8" for the wire exit and insulation filling. Has anyone seen anything like this purchsable through a catalogue or other vendor?