AWOL, I've sent PM re Garmin cover. Moodybod, if you're really stuck for ST50 covers, I think ST60's might fit, not sure though. Can you borrow one to try?
My boat once had a mixture of 50's & 60's. I seem to remember the 60 covers would go on the 50's reasonably firmly, but I might be wrong. The 60 is basically the standard 110 mm square, with the curve adding an extra 5mm.