i made a wheel for a servo motor modified for 360 degrees (for an antweight) , i pressed a wheel sized hole into the plasticine (just ordinary kids playdoh type stuff) and then i pressed a servo horn into the plasticine to leave a servo horn shaped impression . i pressed cling film into all the nooks and crannies , and mixed up some 5 minute epoxy , i casted it and , although the cling film distorted the shape a bit , it fitted the servo ok

I dont know where you would get urethane resin , but i have made tires out of silicone window grout .