Me and my daughter are building a small fixed gantry cnc router. Probably 600mm or smaller. I have some aluminum extrusion that I picked up for free. From what I can tell it was originally used for mailbox posts or deck railing. It's approx 3.5" square. My thoughts were to brace the bottom half of the 'H' with some pieces of 1/4"thick angle iron (free from work) and then fill the rest in with epoxy granite. I would then attach the linear rails to the extruded aluminum in the open side of the H. Would there be a substantial benefit to filling both side's of the aluminum with eg vs filling just half? I can get a lot of angle iron scrap from work. I was thinking about running two pieces of angle iron the entire length and then possibly welding some braces in between them. This would make a primitive frame that I would bolt to the aluminum before casting the eb. Would adding a significant amount of steel add any substantial benefits?
For the table. I have some 1/16 thick pieces of sheet metal. Some of them are already formed into "tubs". I attached some pictures to give a general idea. I was thinking about taking some 1/4" angle iron and building a square "ladder" shaped skeleton. I would then make a shallow "tub" out of the 1/16 steel sheet. I would attach the tub to the steel frame and then fill it in with eg. The steel sheet should be flat enough to mount the rails and then the eg would be the top of the table or vice versa. Would it be better to have the steel sheet for the top or use it to attach the rails on the bottom?

Attached are pictures of the aluminum extrusion and some of the random scrap to give you a idea.