I use the clamps from Altronics (part ALT-PWC-CLAMP), you can find it here:
http://www.altronicsinc.com/pages/weathercenter.html
The clamps are white plastic with a nice toggle bolt fastener. I have one on the outside (I use a carefully crafted dent in the trailer edge for the other support) and two on the inside for storage. Have worked great, lasted well, and look good.