I had one on a kite foiling board a few years ago and despite one corner clearly being 1 mm away from the board(I didn't apply it), the foil mount held.
I tied a chord around the mast and tied it to the leash plug so I wouldn't lose the foil if the foil mount came off.
Foil Mount works. It's best use is an inexpensive entry into foil sports. (But as everyone here will attest, you will dig foiling so just go all in get the real thing.)
It's best to use it on a molded board (like a production sup or windsurf formula board) if you want to wing it. That's because it sticks so strong, a standard surfboard glass job could potentially tear off the bottom of the board before the Foil Mount fails. It sticks! Entry level prone foiling is less continuous force, so a glass job would likely be ok. This is all my guess, having installed and used a couple.
Make sure you check out whether your board bottom is flat or has curves so you can order the right one. They are pretty rigid.
As others have said, it's hard to find a proper donor board for winging that is short with higher volume.
But certainly, if you are looking for an inexpensive entry into foiling, Foil Mount does the job. Keep in mind it is not easy to return the board to it's original state if you choose to. It's doable, but not easy. Foil Mount will sell you a removal tool.
Everyone here will tell you to skip this step and get a real board. I will too.
But I have used the Foil Mount and it works. The company is nice. They came up with th Wizard Hat hardware and other stuff. And they genuinely try to improve the lives of foilers. All good.