I've got the original feet on my PL-560 but when I got it the rubber bladders that live inside the feet and house the suspension springs had all rotted to the point of separating from the plinth. I glued everything back together after a fashion and they work but I've never been really happy with the results.
John, your idea of the dowels and rubber feet gives me an idea for a mod to your basic design. As far as I can see, there's no suspension to speak of. The dowel is firmly attached to the plinth and the feet fit snugly over the dowel. Since the PL-518 has no internal suspension the whole turntable is fairly rigid, yes?
What about replacing the dowel with a length of PVC pipe over which the rubber feet fit closely but can still move freely in and out and then inserting a spring inside the PVC pipe? Next install the rubber feet over the assembled PVC and spring. The spring would have to be long enough not to bottom out from the weight of the table and of a strength to afford the right amount of suspension.
In my case, I'm sure the original Pioneer springs would do the job. All that would be necessary would be to determine the right height for the 4 pieces of PVC pipe to allow for initial compression when the turntable's resting on its feet plus enough room for suspension travel.
I think this would work pretty well but I'd love to have you all shoot holes in it before I make a terrible mistake.
John