I have a strong love of Quakers so I would say get a set of floats on the Flash... 21" sounds good. My calculations say around 22" but 21" is very close. The problem with a lightly loaded airplane like this is that most floats are too bouant. Each float should dispace the weight of the aircraft. In other words if you have a 2 pound plane each float should hold up 2 pounds before sinking. With a plane like this most floats that are the right length are too wide (and heavy) because they are designed for a normal airplane. When I built mine they ended up very narrow and look elegant like the Quaker. They also ended up very light. I built mine up from balsa and ply and used .5 fiberglass to seal them. This is on a 105" Quaker, almost exactly twice the size of the flash, and my floats are 50" long. I would build them a couple of inches shorter if I was to build another set. I've built many out of foam and fiberglassed them and that's what I would recommend for you.