Escape-proof chastity belts
Most people are familiar with chastity belt from sex-shops that are used for BDSM games. Basically it's leather belt that can be pulled out without key or, in worst case, can be cut by ordinary scissors.
Virginity protecting chastity belt must be made from stainless steel. It must be measured and crafted individually to sit tight enough on wearer's body. In this case it can't be cut by a simple bolt-cutter.
Metal chastity belt is not medieval myth, there are number of steel belts manufacturers like Tollyboy or Neosteel.
Among other chastity devices, only chastity belt is secure enough to serve as virginity protector.
Other devices like cages, tubes (in case of male), etc. can be broken or pull out easily. Although broken device will be discovered by another party, but failed to prevent intercourse. In case of virginity protection, no need to find out loss of virginity, but not allow it.