Yes to Stil,DD and adamsmith. We are still awaiting a ruling from Judge George but I am pretty sure you are on the right track.
I think this word is following the Australasian rule of adding 'y' or 'ies' to words, Other examples include 'brekkie', chrissy', 'gummies', 'hottie', and 'rellies'.
In any case, we may well be the first to spot this word in print because I cannot find a reference to it in the usual sources.
The word 'stubbies' is also used in New Zealand. Originally, it referred to a pair of very short shorts and was derived from the Stubbies brand of shorts that became popular in the 1980's. Males of this era would cringe at the thought of anybody wearing them. Later, when small bottles of beer came on to the market they were referred to as 'stubbies.'
While I was typing this George got out of bed to give a definitive ruling on the word.