Basically you get 90% of the rigidity you want, to hold the lathe bend ends in a fixed fixed condition (that is they are rigidly held in 6 degrees of freedom) for just 5 bags of cement for a 5 foot long lathe. And its a tube so the chips will fall off of it onto the floor.I’m admittedly a bit fuzzy on the sono tube shape; are you suggesting the bench top be cylindrical ? I hadn’t really considered that.
You will need to support it on something, so you will need another bag of cement to make some feet on the tube, then stack it on some cinder blocks and bolt it all the way through the stack, to the floor, so it doesnt walk on you when turning something out of balance.
The cement will move a bit for the first month, thats about it.