I'm actually about to make a trailer for my dogs too. I want to be able to use it with my Xtracycle & my Whizzer. I'm using these plans http://www.carryfreedom.com/bamboo.html
I would def say don't use the harbor freight trailer, it's probably too heavy - a well homemade trailer should work fine. Or you can purchase a bikes at work trailer - they're rated up to 300lbs.