I'd like to build a platform for a float in small human-powered parades that can be pulled by a bicycle or people on foot. Most "parade float" info out there is for people building something that'll be pulled by a tractor or other motor vehicle, so I thought I'd put together what I've figured out.
What I Looked At
Lots of floats are built on top of "trailers". Most cheap trailers are two-wheeled, which means they're not stable when they're not being lifted or pulled.
I also looked into "garden wagons" or "utility carts" as platforms for a float. For instance, Home Depot will sell you this thing for $220 (at the time I'm writing this), which claims to hold up to 1,200 lbs. The platform is 30" wide x 46" long and it's got 13" tires. The sides can be taken down and put back on. This was a leading contender for me for a while, as it seemed like the sort of thing that might have other uses? Like for gardening or utility?
I also did some research into farming and wagons, thinking, "Surely if gardeners use small wagons, farmers must use larger ones. What about hay wagons, etc?" This lead me to discover a key piece of terminology: "running gear".
"Running Gear" is what you call the chassis that you build your wagon on top of. It's the wheels, the axles, the thing that sticks out in front that you hook to whatever's pulling, and the frame that connects it all together. It sort of makes sense that you'd manufacture this stuff pretty generically and then let your end user decide what to build on top of it. I didn't grow up with much exposure to farm equipment (or I guess to railroading?) so this wasn't something I knew, but now I realized what I was looking for.
I investigated "used running gears" for sale near Chicago, but perhaps unsurprisingly, the market for used farm equipment is limited here. This is pretty clearly what you're looking for if you want to build something big and on the cheap, but you likely need to have some farming communities nearby and somewhere to keep something large.
Random googling eventually brought me to the above video, where this guy was building something that seemed just the right size. Noteworthy from the video: the running gear itself seemed to come in two relatively small boxes, implying that it's possible to disassemble the thing for storage. Exhaustive inspection of the video revealed that what this guy was building was a "Farm Tuff Utility Trailer", available at Northern Tool in a few different models, ranging from $330 to $770. The version that seems most popular is the lowest-end model, which is rated for 2,200 lbs and has front-wheel steering.
So the Farm-Tuff wagon I chose has 16" wheels and a 48" wheelbase, and can be assembled in a couple different configurations ranging from 49" to 78" long. It was about $320 when I bought it, and I had it shipped to my closest Northern Tool store. Taller and bigger than the garden wagons but can be taken apart for storage. Huge upgrade. It's just a matter of building a sturdy platform on top, sufficient to carry the weight of the actual float stuff. I happened to have some pressure treated lumber left over from a porch project, so I didn't need to buy much wood. Here you can see the frame I decided on, and in the back, the reinforced plywood deck that I'll attach later. I used the "cutting a slot in it" technique demonstrated in the above youtube video to make the frame work. My frame isn't as hefty as one built by the guy above, mostly because I need to keep the weight down so that one or two people can pull the thing. 2,200 lbs is probably too many.
I'll throw a picture in here once it's assembled and painted black. This is a great size for a small float and seems like it could provide a very sturdy platform for a float or wagon that between one and three people could sit/stand on.