The table I mentioned previously is a 4x10 table for a mpcnc. The cees are made to fit inside the receiver channel so just squaring them up, drilling holes, and dropping a couple bolts through should be plenty to form a square, rigid corner. What you'll probably want to look at are two sides as 'open' purlins(described as 'receiver channel' in the link) and two cross-members as 'cee' purlins. Here's a link with the standard sizes and shapes you'll find.
They may not have them at your standard metal yard, but any steel building supply store will be happy to sell you a couple of em'. That sort of construction would be susceptible to some twist, but it should be plenty rigid for a router sled. I think for what you're describing with a router sled, a simple square held together at each corner with 2 m8-ish bolts would be plenty.
