I once saw a prog in the 1980's with Clive James in Communist Russia where the hotels had no plugs either. What he did was to get a squash ball which fit the plug hole perfectly. You could try that or any rubber dog ball.
Darn, I was in the co-op today and I forgot to buy one. Looks like I'm gonna have to stick to the baby sock for a while yet. I prefer showers anyway. Thanks for (most of) the suggestions guys.
you should be able to buy 'universal plugs' - rubber, wide enough to fit any plughole. I got one before going to the Soviet Union in the 1970s, which meant that I didn't have to perform Clive James's silly improvisations. (I bet he knew about the lack of plugs but thought squash balls would make for cuter TV.)
I've used one of those Rubber Jar Opener/Gripper thing-ys - the ones that help you open a jar lid when it's stuck. Just hold it over the drain while filling the tub until you have enough water to hold it in place.