If you spend a lot of time with layout dye you can grind cold rolled steel to be flatter than those aluminum extrusions.
For 500$ you can buy a lincolin 225 or other cheap welder off craigslist and make a bigger machine from steel.
then pour a 1/8th inch thick layer of epoxy on top and scrape it flat, then bolt rails on it, or your hand scraped cold rolled steel plates.
alternatively you can cast the base from cement, then pour epoxy on top, it will level out.. then bolt the steel to the epoxy.