Got my plastic BCB crusader cup lid a few days ago. It certainly works well - the lid on the cup speeds up the boiling time and at £2.50 it seems a good bargain (then again, it's just a bit of moulded plastic), I can now boil half a pint of water with half an army issue hexy tab in 6-7 minutes and that's to a rolling boil (now I have to buy my own hexy tabs I'm more concerned about economy, but maybe that's just the Yorkshire spirit coming out in me)...
...but I don't know whether my crusader mug is a bit bent, but I have to force the lid on quite hard to get it to sit in the mug - and when it has boiled and I want to get the lid off, the fit is so tight that I have to poke a stick under the edge of the lid to 'ping' it off (and more often than not, the lid then falls into the mug)...
...so if it breaks, I'm not sure that I'll buy another one...
...but if those metal ones that Warthog makes are a loose fit over the top of the mug then I'd be tempted. Or I may just use a piece of thick foil from a pie dish.
Geoff