My X-axis/base will be made from 8020 extrusions that I got from ebay. I did not have a specific size in mind. I happen to won an auction with 13 pieces of 1530 with length of 27.75" each ($125 for all). I then also won 2 piece of 1530 @ 50" each. There's some really good deals on ebay for aluminum extrusions.
Parts List: (price does not include shipping)
(2) 1530 50" , lightly scratched surplus from 8020 Garage for $55.78
(4) 1530 27.75" , lightly scratched and a few small holes drilled on side for $39
(8) rack clamps from CNCrouterparts.com for $48.00
(8) 5/6" hex bolts 3" from Home Depot for $2.40
(2) 20 pitch 1/2"x1/2" x 72" (cut down to 50") rack for $93 ( could have gone with the 48" length)
The rack clamps came with carriage bolts that holds the rack onto the 8020 using the T-slot as an anchor. The way my frame is designed, there is a cross member (4) on the opposite side of each clamp. I replaced the carriage bolts with 3" hex bolts that go through the X-axis support to the cross member. Each end of the 8020 cross member was tapped for 5/16-18 threads. The trick was to have the holes spaced apart to match the clamp ( 1.5" spacing between holes). I had some slight misalignment so had to enlarge the hole a little to 3/8" on the ones that didn't quite line up. This wasn't a problem and it gave me a little wiggle room to adjust the cross beam so they are squared.