If I understand this right, you are using the HDPE as the rims for you tire. If so, one method you could do is use the four arm servo arm to screw the wheel into the outer hole of each of the arms. Basically, find the center of the rim, place the servo arm centered on the rim and mark the holes for the 4th hole of each part of the arm as well as the center. Then remove the arm and make pilot holes for the 4 outer holes using a 1/16th drill bit. I am not sure of what size to make the center hole, but the idea is to make it large enough to put the servo arm screw through it so you can attach it to the servo. Screw the servo horn onto the rim using the 4 outer holes, but the servo arm on the servo and put the screw back into the servo. You will want to make sure you check the clearance of the screw heads with the servo.
Hope this idea helps,