Cheers, Dave - I ended up going with Roqsolid. Usually their covers drop over the top, but they made me a custom cajon-style case that you put the amp into, and zip up at the top (similar to yours, from the sound of it).
Here it is...
(A little tip for anyone thinking of doing the same... mine came with long stitched-on handles, which I don't need as I'll be carrying it by the amp handle. They'd just flap around and get on my nerves, so I carefully unpicked them and removed them. If I were to order again, I'd specify not to include them.)