Designed this tool as my data is often faster than my house wifi, so I make a data hotspot when I need a fast connection for download etc.
The design is really simple, a conical shape on which you should put an aluminum foil and a base in which you can put your phone, possibly without a case / cover.

The results are better than I expected: an increase of minimum 20% up to 100% in data signal.

You can see my results in the pictures (left: without amplifier, right: with amplifier).

The tool should work for any signal, wifi, data, bluetooth. Make sure you point it at the possible source (door, window, etc.).
If you point it in the opposite direction, the signal might get worse.

Make also sure not to leave it in direct sunlight or the reflected light will make the phone hot.

The phone bracket is 80 x 10 mm, designed for an iphone. You can resize the xy plane to fit your phone.

I will also upload a new prototype version with less material and few minor upgrades, named V2.0 if you want to try it.