Strictly speaking, you'll need a schedule 40 PVC cap to go with the schedule 40 PVC pipe. But as T3_1565 alluded, the week before the Waterloo event, it was realized that it was very difficult to find the proper flag holder parts in the Waterloo area. Few stores even carried the proper pipe, though the yellow, non-schedule-40 CPVC pipe was abundant. Schedule 40 plastic caps were similarly difficult to find in the uncommon 1/2" size.
Since it was just a courtesy for the regional to try to have some extras on hand, the matter was left alone until the Thursday of the event, when it became apparent that a majority of the teams did not have legal flag holders. Most were plugged, crimped or made out of the improper pipe (the narrow CPVC, sometimes capped, but often with copper).
Inquiries were made to several plumbing and hardware stores, and of them, only one had any 1/2" plastic caps at all. They were the yellow CPVC (not legal), and they had three. Those were acquired anyway, and it was decided that teams would have to use a functionally identical flag holder out of some sort of 1/2" plastic pipe. (Details such as type of plastic, cap style, etc. were ignored.) This was the only practical way to ensure that teams passed their inspections.
For the Greater Toronto Regional, spare pipe and caps have been ordered, to avoid a repeat of that incident.
Incidentally, to get an idea of what is available, search
McMaster-Carr for "PVC schedule 40 cap". Part number 4880K51 is a legal cap.