A while back, I decided to try making side bearings for my trucks. I used .012" phosphor-bronze wire (wound around a 1/32" wire for a mandrel) to make the spring. They are anchored to the bolster with 1/32" rivets. The bolster is 3d printed SLS nylon. If I could ever get around to finishing a freight car, I could tell you if they work or not.
(https://www.finescalerr.com/smf/proxy.php?request=http%3A%2F%2Fburlrice.com%2F_LS_PS5077%2Fside_bearing1.jpg&hash=a38c2426610b1e3f9191b88dc4e4ad7426fe90be)
(https://www.finescalerr.com/smf/proxy.php?request=http%3A%2F%2Fburlrice.com%2F_LS_PS5077%2Fside_bearing2.jpg&hash=1157becd688c73f04456283520e1b478e286b411)