Repo for the initial version which was just an eyeballed copy of Wolfgang's design: https://github.com/crteensy/FETProbe_v0

Repo for the "tiny" version with RF shield and power supply/signal through u.Fl: https://github.com/crteensy/FETProbe_tiny

The build is pretty simple, but there are two things to point out:

The input capacitor is not a component you can pick up and place, but it's formed by copper features on the PCB. Here's the top part, a large pad next to the FET's gate 1 (pin 4):

On the bottom we have copper as well, but connected to the through-hole input pad and configurable by some little extra copper pads:

We haven't tried it but theoretically the probe can be tuned a bit with this.

The second thing to note is that the RF shield must be modified prior to soldering, because it would otherwise provide too much stray capacitance and wouldn't really fit either. So one sidewall has to be removed:

I did this by filing the edge off a bit and then wiggling it until it fell off entirely.

However, before the shield can be soldered on, all the other parts have to be soldered. It's not much:

I'm thinking about ways to make it smaller, but that would probably require a custom RF shield. And it works.