Welding Tips and Tricks

Has anyone done any kerf cutting, or scoring of plate to get a sharp outside corner bend? I recently got a nice Amana 8" tablesaw blade set (two pieces, left and right) which cuts a v-groove with a small flat in between. You can then fold plate fairly easily and then weld the inside corner however you might want. In my case I'm leaving about .050" of material and making the fold without any machines etc, just a bit of elbow grease. And that part works great.
So my problem is that i cannot figure out how to clamp/fixture the part after the 90 bend in order to get a good welded result. Seems easy enough with some angle or square tube and a few good clamps, but i've tried loading it with clamps and bars and everything and i can't get it to come out 90.
The parts I'm trying to do are 16" wide (thats how long the bend is) and the plate/sheet is 1/4" 5052. So I've got 16" of run to put down a bunch of short tack welds. I do these about every 3" or so, wait for it to cool, and then fill in between. Now i know thats a lot of weld, 16". But there has got to be a way to get that to come out 90.

The images attached are in a previous test in 3/16" which didn't come out too bad, but with the 1/4", i was unable to keep the 90. Anybody?

You may have to start out with the part not quite bent to a 90. That way after it pulls in it will be 90.