Mailbox flag. Held together and attached with 2 screws/washers/nuts. Not sure exactly which screws I used but the thread diameter was roughly 5mm and head diameter 8mm.

It has a small nub to help keep the flag vertical. With my printer it came out just right, but if it's too hard to slide the flag, you could file it down a bit.

I recommend slicing at 0.2mm layer height.