Drill it with a hole saw or any bit that you have that is the right size. Use a little silicon on both sides of the fittings just to be safe.
I have used hose barb fittings on the outside of the container with a threaded fitting on the inside to serve as a locknut. I matching rubber o-ring on the inside works well too.
ok. i ordered wazzel's float valves and ordered the tubing an shtuff for the r/o unit. I am gonna build the jobber on monday and tues. will post pics... if anything comes to mind... please send me a PM or reply here. i am monitoring the thread..