I made a very rigid lathe bench for my little 7x lathe, filled with concrete.
I had no shrinkage.
Bench is 120 mm high mild steel, 12 mm thick, in a welded box, with crossed 15 mm rebar in center, welded to each other, 3 each of x and y crossing, in the center.
I put a 200-500 kg preload in the box, by heating the rebar with propane, before welding the last end pieces in.
Steel about 50 kg, total mass about 150 kg.
Results about 95%.
Works extremely well.
A bigger 8x lathe would have been better.
Cost about 250€ in materials, in Spain, 2005 or so.