Nunhead use postit notes too (not serialised), and very successfully. They can easily handle the deluge of people waiting outside.
336 use google sheets and forms with a tablet. Looks neat and tidy, but it can be hard to cope with backend changes (you have to re-publish the new sheet), not a big issue - except when you are in a rush, and you have to remember intricacies. The process of putting in the data into google forms is slow, much faster to write john bad laptop and stick it on top of a board, when John gets his turn, the postit moves from the top to the bottom of the board, is both simple and fast. If you offer clothing repair too, use two different colours of posits
I think were the spreadsheet shines is that while the even is running you can sign off the call details as they leave, making the event stats easier, but think 336 use paper, and lets us fixers put write on it (I rarely do, other stuff happening)
Big tip learned from two other cafe’s that I have never visited (Tooting fixers and Rotherhithe Repair Café) vs 336, don’t all have your events on the same day (first Saturday of the month), often hard to get reliable volunteers, and they are often thinly spread. Speak and visit each other, there is no contest between the groups, no ‘repair group of the year’ award, and do choose an alternative day if there are other events held near by. As a fixer at Nunhead, if a part was needed I have told a few guest to buy the parts and “visit me next week in 336, or wait a month”. At 336 I have told the person that this is what you need (from ebay), “get it fixed next week at Putney, I’m sure they will help you, but I do not go to that event.” - too far, one way nearly 2 hours travel for a 2 hour event
Quieter events just have a piece of ruled paper, first come, first served.
The other method I have seen is where the group uses Eventbright.com and book one hour slots - think booking cost £2 if you do not attend, but is refundable if you do (to prevent no-shows). The guest arrives at a pre-determined time. If the fixers previous fix has over run say 30 mins, the group organiser may remind you the next person has already been waiting 30 mins, and asked to hurry up. Personally, to me this feels more like a job than a charitable service, but does prevent a deluge of people, and you get to know the workload in advance
Dont leave your consumables, plugs, fuses, glue etc, in an open area easily accessible to the public, as some see it akin to leaving chocolate on the pillow of a guest house 
Buy you volunteers things to use, like plugs, fuses, glue (super glue, multiple tubes from a pound store or Ali, and don’t put them all out), heatshrink, etc, otherwise they will quickly feel irked that they have to keep dipping into their pockets to buy stuff for guests, or complain to guests, and it also demotivates the other fixers. Don’t ignore your volunteers.