@dcindc plinths for idler wheel turntables require some thought put into their design if you are interested in wrestling the best performance out of your deck. Why? Because these decks (301/401, TD-124, various Lenco models) transmit energy differently than belt or direct drive turntables. I would divide the world of idler wheel plinths into heavy and lightweight. I've had good success with both but I suggest skipping the very heavy obsidian or rock plinths. It's possible to completely isolate the deck from the plinth but it's really hard and that outweighs the benefits IMO.
Chris Harban makes some great plinths - Woodsong Audio. The best plinth I ever had for a TD-124 was one I make from stacked and glued 1/2" marine plywood, carefully braced and damped in key locations. It was a fun weekend project but my wife didn't like the utilitarian look.