You can buy 1'x1' squares of 1/8" stainless sheet steel at home depot in the roofing section for $10. I bought one and made a support for both sides and the little carb blocker since I removed the rear fender. you can see in the pictures. no welds necessary for this one. though I haven't tested it yet, but it seems much stronger than before. I did use an angle grinder a file and some sandpaper. and titanium drill bits to get through the stainless.
*you can make a nice template by placing a piece of cardboard against the sided of the subframe and applying pressure along the edge of the subframe as to dimple the cardboard. then cut it out and trace it on your steel sheet.
** I believe you will have to remove the engine and the rear wheel to properly mount it with nuts and bolts like I did.
*** I have a welder and opted out because the alloy of that subframe is so weak I was affraid it would just melt.