Hi, sorry for the delay. I would go with cast iron also as a first choice. If you were in the states I would say contact Dura-Bar in Woodstock IL. Seeing your in South Africa and it maybe difficult to find pre cast -cast iron I would settle with what you can find. You understand that milling on a machine like that is "lightweight" even without a riser and depending on the thickness you use, it will be less. I have used Cast Iron plate to rise Fellows gear shapers and I mill out the center to lighten the riser. If your only drilling holes and light milling I would think Aluminum would work. If your planing on milling steel then I would use cast iron. It looks like a Chinese or Taiwanese machine. Look at the motor and see if it says. Rich