My homemade hookah (there is a thread of it w/ pictures somewhere, just search if you want to see it) was made using a glass vail, a rubber stoper cap for big water pipes, and four hoses from ace hardware and it works pretty good IMHO.
First things first, make sure you have big enough hoses, we had to replace the hoses twice because they were too small and too long and it was a pain in the ass to get any smoke through them.
2nd, I'd just cut a small hole with scissors and then patch it up with acrylic (every hardware store, and most other stores will have this). It should be a better seal and it will be much easier then trying to melt the plastic.
3rd thing, make sure the downstem and bowl are plenty large, because it is going to be hit by 3 people at the same time, you want to want plenty of room for all that smoke to travel
other then that, have some fun !!!!