Don't think leaving the oval holes will work too well. I don't get much swarf dropping through the bed on my machine so why not fill up the entire sections? Looks a good idea overall to me though. I've just finished filling the column on my small X1 mill with epoxy granite and it really deadens it - haven't checked the stiffness yet. I think I'll try filling the bed of my minilathe sometime in the not too distant future. Are you definitely going to do yours?