For anyone who needs to put vent holes in their tubs but doesn't like the idea of burning them open with a soldering iron, I have a simple solution.

About a million years ago, I used to build and race R/C cars. RC cars have plastic bodies, and must be mounted on the chassis with mounting posts. The holes have to be reamed in each different plastic body. Years ago, I bought a tool designed for reaming perfectly uniform holes in plastic. I've used it on all my tubs, and it works like a charm.

Here's the tool:

http://www.hobbyetc.com/cgi-bin/item.cgi?part_id=44141

These reamers are made by several different companies, and you should be able to find one at any hobby store that specializes in R/C cars and trucks.