So I'm going to put my way on the internet, so it can be acknowledged as being a correct way too. By the way I've sold these barrels to people that don't want to build thier own. I've never had a complaint about my design.
So, lets get started.
First of all. You do not ever want a bulkhead fitting! Not Ever! Why you ask?
1) They are expensive
2) They usually leak or start leaking soon after being installed.
3) To use them you have to cut the top of the barrel open. Which is a bad idea because it causes an evaporation problem. It's also a bad idea because it turns your barrel into a mosquito farm.
Materials for this project are:
1) Food grade plastic barrel. $5-$10
2) 1/2" Hose bib $5.30
3) 2-3" of 2" pvc pipe $2.90 for 2 feet
4) 2" pvc male adapter $1.14
5) 3"x2" pvc coupling $2.98
6) 1/2" nylon male x barbed adapter $.60
7) 4" square of fiberglass window screen material free from my scrap pile
8) A length of 1/2" garden hose, again from my scrap pile.
9) A tube of Rectorseal pipe sealant. Not sure of the cost, less than three bucks and it has built at least six barrels.
Tools needed are:
1) Electric drill
2) 3/4" spade bit
4) Punch or metal rod
Now place the other end of the short pvc pipe into the 2" male adapter. You've now created a funnel to place you downspout into. The screen will keep out mosquitos and also prevent debris from your roof from getting in.