A bar that only exists when the site is full does not need its own till, its own list or its own procedure. What it needs instead, and how to take it down.
Most venues have bars that are permanent and at least one that only exists when the site is full - an overflow counter in a marquee, a bar in a room that is otherwise locked, a trestle at the far end of a field. It sells a short version of the main list, it is staffed by people borrowed from elsewhere, and it is open for two or three days. The temptation is to run it as its own small business for those days: its own till, its own handwritten list of what it sells, its own way of telling whoever is making the drink what to make.
That is the version that causes trouble, and not because the bar is busy. Everything about it that is separate has to be joined back up by hand afterwards - sales sitting outside the site's own figures, a list of items nobody else can see, staff who learned a procedure that applies nowhere else on site.
The framing that works is to stop calling it a second bar and call it one more place an order can be collected. A guest orders from the same menu they would have used anywhere else on site, the items that bar makes are pointed at that bar, and it prints a ticket with the guest's name for collection there. The ordering, the paying and the printing are unchanged; the only new thing is one more destination a ticket can be sent to.
The staff briefing is then short, and that is the real gain. There is nothing bespoke to learn: read the ticket, make what it says, call the name, hand it over. Anybody who has worked a counter elsewhere on the site can work this one - which matters for a bar staffed by whoever is free that weekend.
Name the person who closes it before the weekend starts. A bar that exists for two days has no obvious owner on the last night, which is when somebody has to read its numbers and pack its printer away. Closing it means closing the counter rather than deleting it: its items stop being available, no further tickets reach it, and what it did stays in the record beside every other counter.
That record is worth more than the bar was - it is the evidence for whether the thing should exist again, and how heavily to staff it if it does. Kept this way the temporary bar stops being a project. Next time the site needs one, the work is naming a counter, pointing a few items at it and finding a socket.