That's a strange-looking design that won't work too well in practice; keep drawing until you come up with something better. You aren't doing yourself any favors by sinking the moving table in a built-up frame; unless you want to drastically restrict the size of parts you can make, you'd be better off making a much larger table and raising it above the level of the support structures. Using short sections of T-slot connected end-to-edge makes a weak assembly that's bound to get weaker as it flexes. And no, you can't expect something held together with two screws in a configuration like that to have much strength. Use wide sections of extrusion instead of cobbling together a bunch of narrow ones, and find slides which are compatible with them and can be secured along their entire length, even if you have to import them. Or forget about using T-slot extrusions at all, and build your machine out of regular structural shapes, which are considerably less expensive, especially if you can find short lengths as scrap.