a piece of PTFE tube and seat it in the hot end, place a printed washer over it, then the coupling on top of that, which compresses the tube and the hot end
This is more or less how my printer came stock, except instead of a washer at the top of the heat sink, the opening of the heat sink was only large enough for the filament to pass through, locking the tube against the top of the inside of the heat sink. I modified the heat sink by drilling out the top ever so slightly to allow the tub to run through the opening.
I think I’ll try something similar to the washer idea (I can’t print the washer because, well…) when I get home from school. I’ll attach a thin sheet of metal over the opening of the heat sink and drill a hole just large enough for filament to go through, essentially replacing the bits of metal I’d removed.