To attach them to a keezer, you can take some appropriate sized corner brackets and glue them to the keezer then set the drip tray on top of them. There are several epoxies out there that will handle the weight. I would definitely not drill into the keezer.
Just noticed Bobby's is mounted on a keezer. How did you mount yours. - Dwain
1 minute epoxy between the the corner brackets and the freezer (roughed up the finish a little first). Velcro between the tray and brackets for cleaning removal.