Now I see what you did with those nuts. Look for a "jam nut" instead; they are thin and intended to lock a regular nut in place.one nut will spin, need a 2nd to lock it in place unless you know another trick?
also curious what you replaced the rod sping with.
The spring I used was a generic one from a local hardware store. They have drawers of them in a wide array of sizes, configurations, stiffnesses, etc. Take some basic measurements with you and look for one that feels right. This is only a simple return spring and the exact specs are not critical (even works fine without one).