We used a piece of aluminum flat stock with two hole patterns drilled out: one for the kit chassis on the bottom and one for the AM transmissions on top. You should be able to see an example
here (it's the third image on the page if it doesn't link you directly). We are actually in the process of making some new plates that have slots where we mount to the chassis so we slide the transmissions back and forth for chain tensioning. You will probably also want to design a stiffener plate that spans the width of the chassis and bolts in some fashion to the holes at the tops of the AM transmissions. We made ours out of sheetmetal with the edges turned up for extra strength. In addition, it helps stiffen the chassis and prevents twisting and racking (much like the kitbot stiffener plate that came with the kit the last two years). This plate was installed at last night's meeting (which I didn't attend), but there should be pictures up on Joe Menassa's website soon. Good luck.
-Tom