I don't know, a sliding seat is for trim - not legroom right? When I solo my tandem I don't have a foot brace so I end up using my peli case shoved against a pack. It's sort of okay and I can slide it around depending on which foot I use. A bow is probably a weird shape though and so the case could possibly shift unexpectedly. I'd vote daypack w/ nothing crushable inside.

Or, what about cutting some foam (pink stuff) glued together and shaped to fit? Then cover that with glass and epoxy. Even tape/velcrow it in if you wanted. Not sure how that could be made to be adjustable but I'm sure you could figure something out. The biggest problem I see is the shape of the bow. Forward or back a few inches and the width would change quite a bit too.