I lovingly created the shown tool from a piece of 1/2" thick aluminium in order to hold the lower backgear gears in mesh whilst I set the backgear lever to the upper plunger hole..
The tool has an angle of 35 degrees and I used it to support the smaller of the lower backgear gears, not visible in the second photo, and was able to obtain a very small backlash in the backgear gears.