What a booth needs from a register: an order rung in at the front, and a ticket at the bar or kitchen inside that makes it.
A festival sells food and drink out of several booths at once, and each one is a bar or a kitchen in its own right. A register built for a single room assumes one till, one menu and one pass behind it. On a site you have a beer booth by the main stage, a grill at the far end and a coffee counter near the entrance, all selling from the same menu. What you need from a point of sale is one rule that holds across the whole site: an order rung in at the front of a booth becomes a ticket at the station inside that booth which makes it.
Everything else follows from that. Staff ring the order and take the card in the same motion at the front, the ticket prints where the drink is poured or the food is cooked, and the guest collects at the hatch when their name is called. Nobody calls an order over a shoulder and nobody writes a docket.
The work is mostly naming and assignment, worth doing on paper before any hardware is touched. Walk the site plan and name every booth the way staff already name it out loud - the name on the front of it, not an internal code. Then list the stations inside each booth. A booth with a bar on one side and a grill on the other is two stations with two printers, because the two sides work at different speeds and should never be waiting on each other's paper.
Then assign the menu. Where two booths sell the same thing, each needs its own copy of that item pointed at its own station, so a lager sold at the main stage prints at the main stage and nowhere else. This is the step most sites rush, and it is the one that decides whether a ticket ever appears in the wrong part of the ground. Keep spare paper in the booth rather than in a store across the site, and use ordinary thermal printers of a standard you can buy again, so a knocked or soaked unit is replaced off a shelf during service.
Set two printers up on a trestle the day before and run the four things that go wrong on a site. Ring a mixed order and check each station gets only its own items. Pull the paper out of one printer mid-order, finish the order, reload, and watch the ticket arrive by itself. Send the same order twice and confirm one ticket at each station. Switch a station off completely and see whether anybody is actually told, because a silent booth is the fault that costs you orders. If you also plan to let guests order from a code printed on the booth, ring one that way as well and confirm it produces the same ticket at the same station - that is an option on top of the register, not a second system to run beside it.