Having listened to the dreaming of a lot of realms before I started adventuring I would suggest that giving the town box a share is a bad idea. It tends to harm younger adventurers who need the coin the most as well as penalize travelers who generally do not benefit from a box.
That said I love Locke's idea, and letting the box take care of the rounding errors is always helpful.
I would like to suggest two things.
First - Administration - I think that the box administer should rotate around, I'm happy to create the safe location for the box each Market but it would be good to have the person in charge of accessing the box and providing items from it to migrate around. This keeps it from feeling like a slush fund for the town leadership.
Second - Tracking - We need to maintain a public ledger, a 'come see me when I don't happen to be busy if you want to see what's inside' really isn't a good system for accountability. The box should always be well protected so the advantage of letting everyone see what's in it out weighs any concern from robbers. I suggest a rotating series of scribes to track each market's transactions. I will personally fund this by paying the scribe of the weekend 5 silvers to track the market's happenings and provide an update (either through myself or someone else)
Further I think we should track contribution and keep the amount contributed public, this includes keeping track of what portion is from left-overs from the town split. We should track this by market and an overall 'ledger' and allow those that have contributed to access resources if needed. perhaps at a 2:1 ratio. (IE Zihr has contributed 4000 Coppers worth of gold and production and needs some antidotes for a personal quest he can 'buy' them from the town box for 40 coppers a piece from his donation total, assuming we have any) This also means the box generates a 'profit' the more it's used but allows for a lot of flexibility. All contributions should rate exactly production cost Ritual Scrolls should probably not be for sale but available for use for spellcrafting outside of Town Endeavors.
I really think tracking can make supporting the town box a point of pride (I know I'd be wanting to be the top giver!) and we can use some of the suggested donations from Locke as prizes. In example, who ever donates the most this market wins this hat I made, or stole from Glaive, because that guy has an awesome hat.
The real question that comes to mind is who can give from the town box for non-town projects, for example someone new comes to town and needs 5 small healing potions. Can I give them potions? How about Gerty? Liam maybe? we need to have some rules to decide what's fair game for such uses, or even to determine what the standard is.
Thanks for listening!
-Kallith