The idea of a float is that it will appear to "float" on air - wheels are not visible. The Lowboy trailers are nice and low to the ground creating this magical effect. This type of trailer is often used by lawn maintenance companies who are not very busy this time of year so it should be easy to find one to use. You will need to construct a joist and plywood platform for the deck and some skirting to hide the wheels and sides.
HAY WAGON FLOAT
A Hay Wagon gives you the advantage that it already has a good solid platform. And at this time of year, they are not likely to be missed for a couple weeks. You will need to attach a 2 X 2 framework and sheeting along the sides to a height even with the hub of the axle. For safety sake, you will need to make sure that the trailer has a safety chain, and if a pin is used for attachment, that it has a clevis pin securing the main pin. With either design, think in terms of rapid assembly and disassembly so that your float can be stored compactly under a tarp in a dry place.
As you can see, it is important to get an interested group together early with a design idea. Start looking for materials that you will need. For example, if you are looking to make paper mache structures, start collecting those trays that you carry coffee in, or newspapers. The sooner you start planning, the better your chances are of finding economical material for your project, and the better the final design will be.