Those aluminum binding post plates already have compressible rubber gaskets mounted on them. See the photo below. That should be good enough.
Those binding posts were made to use with those plates. Those red & black plastic rings might seal them in the holes on the plates. See if they fit snug before you mount the plates. If not, use a small bit of silicon sealer on the thread where the nut will sit after you tighten it.
Parts Express does sell compressible gasket tape, probably the same as what comes on those plates. It's made with ⅛" thick closed cell rubber with adhesive on one side. That seals well and does allow you to remove things in the future.
www.parts-express.com
But it never hurts to have a tube of silicon sealer when assembling speaker cabinets, as long as you're sealing something that you never plan to remove. I've used it to seal glued MDF seams inside of cabinets, and to mount port tubes.
