Hey Clive thanks For the Reply. The only problem I have is that I have to fill it before I weld up the second end with my screw plates at the end. I guess I could make a filling screw at each end but I didn't know how the sand and resin flows as the hole wouldn't be large.

The rails are 20 mm Round supported type that I bought before I started to learn it wasn't the way to go but I have made the design with an eye to swapping to Hiwin in the future.