That sounds like a good place to start.
- - - Updated - - -
I saw Flexi-Paint on Facebook and it seems to work well on EVA foam. I sent them a message asking if it will work for HDPE. They did reply to me, saying they would test it out, but I haven't heard more (that was only just this morning...
As the title says, I have a TE HDPE Stormtrooper helmet. I got it back in 2008 or so (can't remember). I originally used some sort of Krylon enamel spray, over the years it all easily flaked and chipped off. I'm talking any sort of handling would easily crack the paint. You couldn't even touch...
Looks good. I had the same issue with finding a cork. I think I bought a bag of assorted corks at Wal-Mart or Michael's...otherwise it was nearly impossible to find the right size. I suggest Gorilla glue to keep it in place.
http://www.therpf.com/f9/saurian-brandy-bottle-star-trek-tos-123553/...