I cut up both an AW11 gearbox mount and an SW20 mounting plate (the bit that bolts to the box and bolts to the mount).
They fit together like this
Quickly tacked them together
Then welded it up properly
Then mounted it to the chassis and gearbox.
Found some scrap 5mm 5083 aluminium in my scrap bin, too thin for the mount, but thick enough to make a test mount/template with. Will make the final thing out of 10mm 4140 steel plate.
Test mount fitted.
Now the motor is fully supported by mounts and it uses all stock standard mounting points on the chassis and motor/gearbox.