I found a couple of used Burley D'Lite trailers (can't remember if that's how they spelled that) on Craigslist for pretty cheap. In my case, that's one per dog. I have two, and the smaller one is about 55lbs himself, and I couldn't fit both of them in one trailer together at the same time.

I modified the trailers slightly by detaching and re-attaching the fabric that forms the child seat (sort of a divider from top to bottom the width of the trailer, two thirds of the way back, held in place by bolts that are holding the whole frame together). I re-attached it further back (just used different frame bolts) so it forms more of a back wall than a seat, giving the dog more room.

Then I flipped each trailer over, wove a nylon strap with buckle around the frame, shoelace style (crossing over forming 4-5 large Xes between the front end and back end to add extra support. Finally I slid an old election lawn sign (coroplast? something like that, search these forums for the stuff to get other neat uses) in between the bottom fabric and the straps, just to help distribute the dog's weight and prevent toenails and weight from puncturing holes in the floor. The lawn signs do shift around a little, so I might duct tape them in place or something.

I have two nice, light trailers that I would hazard a guess could carry more than the 150lbs each that they are rated for (though haven't tested that theory).

As far as a dog-retention system, I have two:

For the larger dog, I have an old leash hooked up to the frame of the trailer and clip it to the harness he wears when out biking with me. It's long enough that he can lie down comfortably, but not jump out. No cover on the trailer because he's too tall sitting upright, and slightly big for lying down the whole time. His head comes well over the top bar of the trailer, getting all the great double-takes as we bike around.

For the smaller (but still > beagle) sized dog, the screen that covers the trailer and snaps in place seems to work best. He can stand up, sit down, move around (active little bugger) all without falling/hopping out. The one weakness is the possibility of working his way out the back by pushing aside my altered seat/wall, but except for a head poking out every once in a while, that hasn't been an issue yet, so I haven't given it much thought.

If you choose to go this route, my only regret is that one of the trailers is not the more recent prototype (of the last few years) that has a couple of quick-release wheels and the frame folds down pretty well for storage. Even the towing tongue folds up. The older style is the same exact size, but a beast to store when not in use.