I made a cheap home-made one from Lowes parts, but it looked cheap and homemade and wasn't rainproof. And it shorted out in one season. Agree with Dr. Bob multiple adapters give more flexibility.
I got this this one with LED, very well made and I'm real happy with it. It's much more substantial and heavy-duty than it looks in the picture. $37 with free shipping is not much more than the parts cost (the marine end is an L5-30R). I tell folks I made it in an hour with parts I had laying around in the garage and they are always very impressed.
http://www.amazon.com/Conntek-14355-Pig ... ds=conntek
Cheers!
John