I am also too cheap to buy the backer plate, so i made one. I use a piece of corian turned round and drilled and tapped to fit my stopper mandrel. It has three tapped holes. One for center, and two holes for design ideas and string holes.
I don't use polish on my PR, unless you count Plast-X plastic scratch remover. I start with 600 grit sandpaper, utilizing the AWESOME tip added about the soapy water. Then i soapywetsand with all the MM.. starting with 1500 and going thru 12k. After that, i usually hit them with the beall first two wheels, no wax.. then a bit of Plast-X. They shine like a new penny after that.
Once i got too excited with the trip wheel and wore down the top surface a tiny bit. It was a really neat thing that i did because the material wore down differently according to coloring, so there were these "veins" that you could feel. It was really wild. I am going to try it again and see if it happens the same.
The hole is a topic of interest for me. I used to turn the hole into the piece, but i found it added alot of time and energy to an otherwise simple and quick project. Now i finish the entire pendant, then use my drill press to drill a small hole near one edge for a jumpring. I think the jumprings make them look more professional anyway, so it is a double win! No sanding the tedious string hole!!! BUT... sometimes, if it is for a special person, i will turn the hole from the front, and sand polish it from the front as well. After i remove it from the lathe, i hand sand the back side sharp edge around the hole.
Just like all turning, there are probably 15 different ways to do it and come up with the same outcome. Hope all that helps!!