I drew up an idea for a hood support that looks pretty good:
The thick blue is roughly the line along which the bonnet meets the frame. The hinge is 31cm below the line of the top frame rail. The dark read lines represent the hood stop rod at different points along its travel.
This has the advantage that is is automatically actuated, although it is more complex than a simple prop rod. I looked at doing some sort of telescoping rod, but the hood swing makes it very difficult. I don’t like the idea that it will flap around uncontrollably when the bonnet is closed. I may need to resort to a traditional prop rod.