I made one from the following parts list for less than $150.
2) used golf cart wheels and tires $40 (Craigslist)
6) 1-1/2" schedule 80 pvc pipe flanges $68.46 (Grainger)
1) 1-1/2" x 10' schedule 80 pvc pipe $20.94 (Grainger)
1/2" x 2-1/2" Hex Bolts, Nuts, Cut Washers, Lock Washers $15 (Home Depot)
Pictures at: https://plus.google.com/u/0/photos/106762135861927301326/albums/6106948507821485057
You can get these flanges and pipe cheaper, but I liked the help at Grainger, you can do store pickup there. The 1-1/2" flanges are on a 4" bolt circle and fit the wheel lugs perfectly. Used golf cart wheels and tires can be bough for $5 but these are 10.5" wide. The 10' pipe allows me to support a wider beam catamaran. The whole thing breaks down without tools for hauling on the cat trailer.
I used a rotary rasp in a 3/8" drill to take the shoulder off the inside of the flanges and rounded them out with a rolled up 80 grit sander belt so they free spin on the pipe, the only other tool needed was a 3/4 " socket to bolt the flanges and hubs together. Just wiggle them around on the axle so they free spin well when you tighten the flanges. The 1/2" bolts are overkill but they fit snug and keep everything alighed well. You could probably use nylon 1/2" bolts just as well to save weight, but the entire assembly is not that heavy.
The only flaw is the flex in the span between the wheels causing an arch in the axle with load on the outside of the wheels; but If I keep the wheels close to the inboard of the hulls it should be fine, if not I can add stiffness inside of the axle with a 1-1/2" wooden dowel.
I havent tried it with a catameran yet but I plan on testing it as soon as I get my hobie out of storage. I'll let you know how it turns out!