Get an anemometer to activate a solenoid bolt and as low-tech as possible
This will be an extremely challenging project, especially when you add the solar/battery concern.
Honestly you'd be better off just manually locking down the object except when it is in use, and perhaps using electronics to remind the user to do so (annoyance beeper, etc).
If you really want to build a gadget, then while analog solutions using comparators, etc are theoretically possible in practice it has been decades since such a solution would be a wise choice.  Instead, a sensible modern solution would use a low power microcontroller.  This would periodically wake up, measure the wind speed, do some time average filtering and make a decision before going back to sleep.  Getting an MCU project to run at lower power is a challenge with many potential pitfalls, but it is something routinely done by careful engineers.
A solenoid is probably not the locking mechanism you want, if you wish to be economical of power.  Instead, you probably want some sort of gearmotor-driven lock or brake, such that it will cost some power to activate it, but zero power while at rest in either locked or unlocked position.  Software will manage running the motor until you detect it has complete the change of position.  You'll need a battery system to save up energy from weak solar input for periodic actuation.   Possibly it will be simpler to just use a primary (disposable) battery solution and not bother with the solar which adds a lot of design complexity and may in unsophisticated usage never pay off the energy cost of its fabrication.