Look at how the fullscale Cub tailgear is made. It has a spring steel tailwheel 'strut' and the tailwheel has 'horns'. The horns are connected to springs. The other end of the springs attach to 'horns' on the rudder. You can duplicate the rudder horns with music wire, brass tubes flattened on one end and drilled to accept springs all 'fleshed out' with epoxolite or JB Weld. There are some commercial items available for the tailwheel itself. I have made them from music wire silver soldered to a modified wheel collar. I groove around the wheel collar so that the music wire will wrap around the collar for soldering. The wheel itself is mounted to music wire and the wheel fits over a brass tube. This brass tube has holes drilled to accept a 'cotter pin' for wheel retention. Slip another wheel collar over then solder a brass tiller to the end after inserting this assembly into strut. Once soldered you can adjust the free wheel collar to suit how much vertical 'play' you want. (Paint as needed before final assembly)