This is what I made up for mine as a temporary fix when I made mine up.
Stainless plate to the standard engine mount. Held with a stainless strap. I did this at both ends of the box as I had the box made to the same dimensions as the originals. Once It was all correctly aligned I welded the plates to the box as per originals. (It had passed the MoT with the temporary straps though)
It looks as though you have made a mistake in not using the the original as a size guide so you can only do this at one end. The other end will have to have a plate to the pipe instead.
The only problem I had was with the tailpipe having too much mass vibrating for the thin sheet that the end of the box was made of (it started to crack), I had to weld a thicker circle of stainless to the end to cure it. Worth remembering when you have a box made.