I can only imagine you'd be making them as a joke gift, I agree with others they really can't be comfortable because...well...um... never mind.
But if you must, it shouldn't be hard to find a pair of knitted shorts. There are many in woman's sizes. You might have to size up for them to fit a man.
Next, I found several banana cozy patterns. It'd just be a matter of figuring out where to place a hole in the shorts to transition to knitting in the round for the elephant trunk.
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Marilyn
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Mirl56 at Ravelry, too.
Learn from the mistakes of others. You can’t live long enough to make them all yourself. Eleanor Roosevelt
ROFL! ...too funny! (and very risque indeed, for a fuddy knitting forum, haha.
and yet, I do appreciate your wanting to find a pattern for them. Even if just for a good laugh on Xmas day when everyone opens presents in front of everyone else. Will he be opening them in front of his in-laws? and hopefully no children in the room at the time... (oh....naughty naughty).