If you happen to know someone (or come across and ambulance or someone who works in an ER) May I suggest asking for a 'blood tube'. I draw a lot of 'labs' enroute to the hospital and found that I carry 3 (of the 4) that are just the right size. These 3 are also plastic. They have plastic tops that 'hold' the rubber stopper in place. The ones I use are Purple top, Green top and Blue top. You can easily pull the plastic tops off (and with it comes the rubber stopper). If you want you can remove the rubber stopper and just use that to 'close' the tube or you can reuse the entire top. Either way it 'reseals' well (enough to be dunkable and keep things dry for quite awhile) (I haven't tried the dunk test for time duration so I won't say how long it'd be good for). Not massive in size but if you ask nicely I'm sure you could get a few at no charge.