You're just full of good ideas Jen ..
I'll add it to the option list but the concrete is free down in the garage, just need a suitable sized tin. I'll drop a piece of steel pipe through the middle with a threaded hole one side so I can lock it on the CW bar.
Eureka !!
Just found the perfect mold !! I burn hundreds of CDs\DVDs at work, we buy them in bulk in nice deep cylindrical smooth plastic containers with centring holes etc all molded in place. Couple of those will do the trick nicely.