I saw a pretty cool instructable about making a fabric gig bag that would probably be pretty easy to modify for a mandolin using the same principles. You could probably waterproof it and seal the seams, too. There are a lot of simple ways to waterproof stuff you might also find.
I use a padded racquetball racket soft case like a gig bag. My POS A-style fits perfectly and it even has a zipper pouch for strings and sheet music. Been using it for a few years of hitching and freight-riding and it's worked really well.