BLUEMOMENT

Lost two full length battens last friday.
On board, I had short battens from an old mainsail

How to support the roach for the rest of the weekend:
overlap two battens and bind together/secure with duct tape
slide into the batten pocket and mark the inboard end
stitch the batten pocket closed at that point with two rows of stitching
insert the battens and tension as normal

simples....

for years I've been resisting the pressure from my wife to throw out the short battens or leave them at home. this increases the satisfaction of making the repair!

Or carry a couple of lengths of suitable wood or plastic as big as your longest batten. These can then be quickly cut to the required length & used imediately.

Plastic conduit capping & broad flat softwood beading are suitable as cheap spare battens.

Some stack packs have what amounts to full length battens in them if the importance is worth the sacrifice.
Otherwise , as the man says - keep the old ones in the boat and not the shed - like me... It's just that there doesn't seem to be a good secondary use for them.(aboard).