There are at least seventeen other ways to do everything I mention here.
If your floats have a flat transom like a ski boat, it's OK to screw the water rudder assembly to it. A smear of silicone before you tighten the screws will keep things dry. $6 for a medium-sized ERNST water rudder will be a good investment. If there is no reinforcement to the transom of the float, epoxy on a plywood plate first.
If your floats look like this
Then you have to get creative.
Probably easiest in this case would be a tailwheel bracket with the long extended rudder you mention.
Some people epoxy a tube vertically thru these floats and use a rudder mounted underneath.
Some people have built up a transom (like the bracket for a trolling motor) to mount a rudder to one of these.
Steerng: Remove your nosewheel assembly and replace all the pieces but the strut with a small piece of the same diameter music wire. If the nose wheel tiller on the alpha is below the firewall, then just attach the linkage to that. If the nosewheel tiller is above the bottom of the firewall, get another one at the hobby shop and mount it just below the bottom of the plane. Then you can run two pieces of fishing line back to the water rudder (my preferred method) or a single nyrod pushrod.
Here's a good shot of the NYROD linkage
You don't need 2 rudders. THey just get misaligned with each other like the plane in the pic.