Despite the ease of the number-covering method described last week, several of the open dancer boys I know, including my husband, don't use page protectors for their numbers. Instead, they cover them in packing tape.
The procedure is to cut a strip of clear packing tape longer than the number, then slowly and smoothly lay it across the number. Wrap the excess around to the back. Repeat with additional strips, slightly overlapping them so the card stock is completely covered in the front.
As the person typically trying to pin these numbers to a tailsuit, I can attest that covering them in packing tape makes them strong. It is also time-consuming, so it might not be practical if you arrive late to an event or have multiple numbers to keep track of. But if you want a shiny number that will last through several rounds, bring the packing tape.