sorry dont have any pics but,3 ways come to mind.
first all need to have the out flow at the mast
under the gooseneck.
1#pvc guttering riveted on either side of the boom.
2# lazy jacks with built in sail pocket/bag.
3# triangular canvas
pocket rigged 3 ways to standing rigging
and boom kicker
,also need to drop sail slightly when a squall comes tp create a bag
have also in the past dropped the topping lift
,lowering the end of the boom and dropped sail so can collect water at the mainsheet end.
first two methods are best as dont have to mess with sails!
we have permernently rigged roof with drains over cockpit
so catch rain that way