Are you looking for a table that enlarges the entire original table (e.g. an overlay table) or are you simply looking to extend a specific side. For circle cutting I made a 2'x2' table that attaches to the right-hand side (what the uninitiated might call the 'front') that attaches to aluminum t-track mounted to the side of the cast iron table. The t-track is bolted to the original table with 3 bolts and the table is secured to the t-track with two 1/4"-20 bolts and t-nuts and a single removable 2x4 that serves as a support leg (essentially the table is supported at 3 points).
Although it is a bit buried at the moment, it was designed to be easily and quickly broken down and reinstalled as needed and was customized to mount my modified Carter circle-cutting jig accuaretely and repeatably.
If you want to create an overlay style table to enlarge the original table on all 3 sides, then you just need to take some 3/4" MDF cut to whatever size you wish your enlarged table to be and then mount a few blocks on the bottom side the same thickness as your cast iron table and then add a pivoting block that you can rotate into position to lock the auxilliary table to the underside of the cast iron table (or you could just use magnets, depending on what sort of forces you expect to encounter/resist). If you enlarge it enough that you need to stiffen the MDF, some 1x2 nailed/stapled around the perimeter of the MDF table will provide all the stiffening you are ever likely to need.
If you need photos, let me know -- I just don't look forward to unburrying it at the moment. :wink_smil At the moment I'm trying to focus on installing some temporary copper plumbing to distribute air from my new shop air compressor (a story in itself).