Can a boil and bite mouthguard act as a retainer?

NO: Boil and bite mouthguards fit poorly and therefore would not retain your teeth in their improved position. Your orthodontic treatment will have then been wasted time as your teeth will shift out of their improved position. Retainers may be removable or bonded wire to the "tongue-side) of the teeth have your dentist construct precise fitting retainers to keep your teeth in their new position.

Waste of $$$: So much time and effort was placed in your orthodontic care... Did you loose your retainer? Does it fit poorly? Have one professionally made to protect your investment.

No way: Not rigid enough, but bette than nothing.

No: Would an ace bandage work as well as a cast on a broken leg? DIY Dentistry is worth less than what you pay for it as it can actually do harm. Please have your problem addressed by a competent fully trained and experienced practitioner.