There is a way to stop a crack. Drill a hole at the end of the crack and then it has nowhere to go! Ingenious when you think about it. This was demonstrated on Tomorrow's World back in the early 1980s, IIRC, using perspex under stress to show the pattern of stress with a crack and with the crack terminated at each end using a hole. The technique was used undersea on oil rigs.
Of course, this is not practical on instruments cover, therefore you'd probably be better putting in a telephone call to Kevin at Parts ASAP down in Bristol, tomorrow. They might have one from a breaker that you could have. They'll want money in exchange, though.
Hope you get it fixed.