If you really must mount your pump to the fork, you can use zip ties and a fourth hand tool like the one shown below to attach your pump mount. The fourth hand tool can get the zip ties extremely tight. I've used this method to mount bottle cages and pump mounting brackets very securely many times. However the only things I ever mounted to a fork this way is are sensors for wheel mounted cyclometer magnets.

https://smile.amazon.com/IceToolz-Fo...%2C1213&sr=8-9