To adjust clamping height with my clamps, I push the stake into the ground at an angle, instead of vertically, keeping the jaws of that stake's peg facing either up or down. This means that the second stake, clamping the subject/flower stem, can be held at any height from ground level up to about 425mm, depending on what angle the first stake is pushed in at. (If required, a slight twist of the ground stake will then adjust clamp height a few centimetres up or down, without affecting rigidity).
In comparison, Geoff F's, telescopic clamp can be fine tuned for height adjustment, and is very compact, so should fit inside most camera bags.
Horses for courses me thinks
PS: Does anyone else have any other useful home-made photography accessories to share?
Regards
Mike.