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.
I have used a travel bag made by Overboard on the luggage rack. It is designed for use in boats and is waterproof. It is also low profile so does not obscure the high level brake light.
I have various solutions depending on the trip. For lightweight, short stays I use a Bootbag. For longer trips a Pelikan case (though the one I have at present is too large/heavy).
Graham (G4FUJ)
Sold L44FOR 4/4 Giallo Fly '09 Gen2 MINI Cooper ragtop '90 LR 90 SW
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
Tim H. 1986 4/4 VVTi Sport, 2002 LR Defender, 2022 Mini Cooper SE
The 'oyster' clamshell where the two halves overlap at the join are brilliant and keep out dust and moisture.
I have never been in a position where one is submerged thankfully, but African dust is pervasive and the oyster keeps it out.
But at risk of repetition, not all Samsonites have oyster closures. Look for hard fiberglass i(US spelling!) with interlocking closures. They are brilliant.