How about something like a hasp and staple for locking shed doors, weld an extension into the middle of the long part , bend it round a bit of 3 inch bar or baked bean tin, glue a load of foam inside it and bolt to wheel arch ? When you close it to lock it the foam should hold it secure. Amazon product
If you have internal ribs on your roof just pop rivet some velcro to them, if not then you could go along the side seam and over the door to windscreen...
Trailer. You could fashion a Heath Robinson trailer using a metal pale and mop handle to tie off the bow storing the arrows in the pale next. Then rig up an old HGV twin axle with wheels to weld the pale to.
Think I’ve worked it out. Quick fist bracket screwed to top of bulkhead behind cubby box. Bar between folded up rear seat legs with another fist on top.
Excuse my total lack of knowledge, but .... what about getting a piece if 'drain pipe and fixing that up in the roof inside or out on the roof somewhere and then capping off both ends but making one cap removable so as to put the bow inside, safe and snug.
Right, don't chase me, I'm going of my own fee accord now before you say it.