It's 6" s beam, but you could use pretty much anything. I didn't want the bottom flange to get in the way of clamping, and the height works well for tightening the bolts.

I was working alone when I made it, and my mill was in a bad spot to deal with the long work with no means to relocate it at the time. I'd also have to have built an outboard support stand, but made the executive decision to abuse my shins and get the table done rather than make another item that wouldn't likely ever be needed again (I don't deal with machining long heavy stock).

The channel is 6" as well. It works out to be perfect for C clamps as no point on the table is over 3" from an edge.

In the time it would've taken to make a stand - I had the holes drilled to size. The only difficult part of drilling is when the uneven side of the flange catches and smashes the side handle into your shin (gotta love milwaukee hole shooters torque). I need a mag drill about once a year, but that's not enough to justify the acquisition cost. Renting one would've taken the same amount of time as drilling manually, so there again I didn't lose anything but some bruising. lol My shins are fine now. Been enjoying the table for over a year.

Now that my bridge crane (also shop built) is up and functional, things would be much different. I could support the floating end with the hoist and have no need for a stand. It's amazing how often that crane comes in handy. It's also why there's no wheels on the fab table anymore - I just pick it up and move it as necessary (just like everything but my cnc lathe).