Gearbox i have.

dave its gunna be great when you have done it and be well worth it.
From memory tho the H plate needs to be as thin and as strong as you can get, so prob best out of a metal as made a few out of lots diff plastics and they just snapped.
keep that in mind tho ok mate, better to spend the time from the start on that bit nothing worse and it snaps and have to make it again as it can be time consuming.
make out of cardboard first for template thats what we did, and tested that first before we even cut a thing out.
just trying to save you some time like you did for me otherday
It has to survive a miss shift.

I might also recommend High density polyethylene (HDPE). Very tuff stuff and really makes it easier to shift between gears. It usually comes in 1/4" (6mm) but you can also double it up if you are worried about wear. It will not break like acrylic or other plastics.

Infact got a better idea go find a ferrari dealer and say the H shift plate missing on this model i don't want it now bye ! lol

lol, that used to happen at a Chrysler dealership that I worked at when I was a teenager. The Dodge Ram had a huge rams head hood ornament, it would to out for a test drive with the ornament and come back without....the customer would say "It must have fell off".

infact i got an even better idea than my last idea, leave the H shift plate and take the ferrari lol

Good idea, there is a plastic manufacturer nearby me that I can by plastic HDPE sheets from.
Ive got kids to deal with today, so maybe tomorrow I make a start.

I went to hardware store to find a suitable ball spring and some form of catch.

I found a 18mm door ball catch and some small draw handles. It looks like a possibility in setting up.
Catch ball $8 , handles $5 . @f1iceman

You could use a piece of Flat steel (or RHS) and drill 19-20mm holes all along it so the ball engages into that to give you the notchy feel?

Good thinking 99!