I asked a local commercial welder to put mine in. He does contract work for DOD suppliers and is very up to spec. The guy was leery of the job but did it anyway with great success.
First he ground the divider to fit the opening. Then, instead of running a bead all around the long parts, like you would with caulking, say, he nicked off small triangles from either side of the top and the tip of the bottom leg at the outside. They were maybe 1/4" but no more than 3/8" deep. When it fit nice and tight, he stick welded it in those three places and filed whatever stuck out with a flat file.
The trick was that the divider sat just a little bit proud of the surrounding surface. Something less than 1/32" and no more. Possibly 1/64". That held it in place if the welds should break. They never did, as far as I know. Because I never had any trouble with the thing. The pressure of the exhaustmanifold and gasket held it in place.
Funny that: all those guys up there did the same thing as my guy -- and my guy had never seen the thing before.