Well if you go into a branch of Parkers then you can get one.
They are actually the lower headgasket retaining seal sump plug washer for a 1958 Austin Morris ( Morris Oxford III and not the Minor) which is still in production in India (the Ambassador) so you should have no touble finding one.
Of course you will need to shave down the scalloped edges a bit but a spare Bovie should sort that no problems.
Finally you will need some way of making sure it stays attached but a small blob of Pink should be enough.
or of course as it is issued then go get a new one from the stores bloke - your choice.
Flasks are on issue these days??? worlds gone bloody soft, what's wrong with a tin mug (enamel, white, drinking, for the use of) kids these days don't know their born,,,,,,,,,,mumble, mumble.....pisses kecks due to old age ....SLR, that's all you need......they don't like it up 'em.......mumble, mumble