We always use a Samsonite suitcase, wrapped in a waterproofed groundsheet.
You could get a specific waterproof covers made up; try a chandler who will give you the name of a local boat cover maker.
When Jane's with me, we use a Samsonite case, but as it's got lip seals all round, we've never bothered with a waterproof cover and never needed one, despite going through some extremely heavy rain.
On my own I just use a 25 litre Peli case
