Pretty sure few of the above responding posters, if any, have owned a Perception Sonoma. 
So - the manufacturer advises against a drain hole. I guess that they think the hole would necessarily be so close to the seam as to invite a fracture. Probably best to heed their warning.
Dump & pump that you are doing is probably about the best option - although you could use a bailing scoop (made from empty bleach bottle) and sponge.