Not as good as the proper thing but i remember using the tops from bottled water as an emergency stopper, obviously you've got to find the right size bottle tops.
don't know if you managed t get some bungs? last time I bought some I was told they were getting very difficult to source, so ive changed to using these. they cost a little more and you have to make some adaptions, but they work much better
from polymarine.com Budget Valve complete PVC base, Black
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38.73.10 - Budget Valve complete PVC base. Black plastic low cost simple and effective complete inflate/deflate valve
I`ve been using DRENCH Drinks screw caps for a long time now, be careful as there are 2 x sizes, they`re quite a common drink so you shouldn`t find it hard to find, they work a treat !!