My favorite place for cab plans is the Bill Fitzmaurice Loudspeaker Design website:
http://www.billfitzmaurice.com/
Designs go from fairly simple (Simplex cabs) to fairly difficult (DR cabs).
It was actually the help that I got from the forum associated with that site that lead me here. When I decided to try to build a kit guitar, I wanted the same sort of help that I got from that site. The help there is phenomenal, and it does not hurt that the owner is a bonafide audio engineer.
My main cab for Bass is one of his DR250's. It's one of the hardest to build, but it's a fantastic cab. It's as loud as an average 4x10, but weights 20 lbs less than a G&K Neo.
When I play outside and want extra oomph I pair it with one of his Titan 39 subwoofers. That gives me similar output to an Ampeg 8x10. The difference is that I can carry both cabs. With my back I'd need a forklift for the Ampeg cab.
Still questing for the lightest gig-able bass cabs...I have plans and plywood to build a couple of his Jack 12-Light cabs. These use thin plywood and aircraft bracing for stiffness. They should be louder than most 112 cabs, and weigh in at 20 lbs.
I have been really impressed by these cabs. I started building them because I wanted light and loud, and figured I could EQ to get the sound I wanted... But the reality is that I really like the way they sound.