Hey Sundragon, I can't think of any sites that will give you a detailed set of plans, but you can find some good info, and ask questions at:

http://www.webervst.com

things to consider are, required ohm load of your amp, power handling capacity of your speakers, ( was that 4x120wx8ohm speakers or a 4x12, 120w, 8ohm cab?), the wrong ohm load will blow your amp, not enough power capacity in the speakers will blow the speakers.......

Dimensions and Design, is largely up to you and your size/style requirements, I usally try to find something similar to what I want and get some ideas from that, then get out the graph paper and pencil..............

Materials and Tools, I would think a bare minumum of 1&1/2 sheets of 3/4inch cabinent grade plywood, birch or apple ply, lots of 1&1/2inch drywall screws, a hand drill, a countersinking bit, a tablesaw, a router and an electric coping saw if you got one, 18ga wire, connectors, 2 or 3 input jacks, to finish up, some tolex, speaker grille cloth, some metal corners and some handles, have fun.....