After looking through some of my XX spare parts, the solution quickly came to me and the original XX rear pivot plate, part number A-2110 provided me with the answer.
Here's a walk-through with some photo's for anyone that wants to have a go.....
You will need the following:
1x A-2110 XX Rear Pivot Plate / Support
an X-Acto Razor Saw
a hobby knife or scalpel
240-grit emery paper (what I had available)
800- to 1000-grit (super fine) emery paper
Use the X-Acto Razor Saw to remove the rear section of the A-2110 pivot plate. You need to remove the rear 'tab' with the two countersunk holes by cutting inside where the transmission case sits. You'll then need to cut and trim the excess material back until the correct size is achieved. If you don't do it this way and just cut it straight off, you won't be left with enough material around the countersunk holes for the screws - hopefully this will become apparent. Obviously the rear pivot plate will be sacrificed as it cannot be used again.
Once you've done the hard work with the X-Acto Razor saw in removing the rear tab, use a hobby knife (be careful

Here's what it should look like when you're done removing all the excess material.
You'll want to leave enough material so that you can create a 45 deg. chamfered edge so that the brace can clear the bottom of the shock tower when it's all screwed into place. This photo should help clarify this...
...and here it is in place. When you're happy with the result, use the super-fine emery to clean and polish it up to make all the sanding marks disappear. How much time you spend doing this is up to you but if you're a perfectionist like me, then probably too long even though you never see it anyway!
It took me a few, very satisfying, hours to do but it fits perfectly and should help prolong the life of the transmission case as we know they are very hard to find these days.
I know there are other transmission brace's out there from the likes of Fibre-Lyte, Trinity, Jammin' and maybe one or two others but this is probably the cheapest solution and as an added bonus, has a little bit of flex it in it which the others do not have.

Anyway, I hope you find this little mod useful. If you have any questions, I'll be happy to answer them for you.
Enjoy!