Looking to cut down on clutter in the house I decided to get rid of all my cheap guitar pedals and amps and go direct to the computer. Investigation online and experimentation led me to believe that unless I spent €€€ on a decent USB interface then it wouldn't be worth it.
The major issue is that cheap soundcards are designed to drive electret mics so if you plug in a dynamic mic or passive guitar then the sound ends up low and distorted in a bad way. This is because an electret mic is actually a tiny circuit containing its own (sort of) amplifier so it expects the computer to feed it some current (a milliamp or so) and outputs a strong signal, probably only at a few microamps. Even though it is possible to modify the cheap USB soundcard to stop it feeding current into the guitar, the signal from passive guitar pickups is too low compared to an electret and will end up very noisy after the software amplifies it to an audible level.
With this in mind I set about finding an amplifier that would sit between the guitar and the soundcard to bring the level up to a more useful range. After blocking the electret drive current, my old 4 track worked but noise was still a problem. I bought a proper (but non USB) interface from a music shop which was unfortunately even worse!

What was the issue? Given the proper interface failed I was sure the levels and components were matched. Then it hit me! I had been going about the problem all wrong, by trying to turn the computer into a guitar amplifier but why not try to make the guitar into a computer (electret) microphone? This approach ended up far cheaper and far better sound quality than I expected!

Supplies

Bag of assorted resistors
A film capacitor about 100nF
A couple of potentiometers for test purposes
NPN audio transistor (i.e. S8050)
Small plank of wood
Some screws and washers (I used countersunk types but normal washers should work)
Some bits of wire
A computer
A computer microphone cable or broken computer microphone
Your guitar

Step 1: The Circuit

I won't describe the electret circuit in detail because I will use a slightly different circuit for the guitar but the premise is the same - the electret circuit takes a small current from the computer which makes it work and outputs a voltage signal. The key thing to understand with this type of circuit is it is half inside the microphone and half inside the computer itself so when the two are connected up, the whole thing works properly.
The unfortunate thing is it is quite difficult to look inside the computer to see what the computer half of the circuit is, however we can guess based on common knowledge that a sound blaster type sound card will output 5v at about 2.5mA. The electret side of the circuit can be considered a very high impedance circuit so a guess at the computer resistance (not impedance!) can be made as 5v/2.5mA = 2kOhm
we can also guess that the audio is taken from the electret as a decoupled signal directly at the input so a guess at the computer half of the circuit is a 5v supply going through a 2k resistor which is connected to both the input plug and also internally through a decoupling capacitor, to the sound card.

Step 2: Testing

All credit goes to Hoofbags on YT for this idea! Use the piece of wood and screws and washers to assemble the circuit, allowing you to unscrew things and try different values for things!
The circuit as drawn shows the values I obtained by using potentiometers in the locations but the final circuit was assembled using the nearest value resistors I could find.
While testing, I recorded what the computer heard so I could use a sound editing program and look at the actual sound waves it had recorded. This allowed me to find the best circuit for my guitar which would not overload the sound card (causing clipping of the signal) but also retained a low noise level.

Step 3: Circuit Board

The circuit board was designed to go into a small enclosure but I ended up cutting it down slightly so it fits inside the guitar and a toggle switch controls if it is on or not.

Step 4: Conclusion

The circuit fulfils my specific requirement perfectly: reducing clutter in my house greatly. The noise floor is just below -50db which is enough for my use and although the circuit is not hifi by any means, it does the job in a pleasing way when combined with my cheap guitar, I think because it is so simple.
You might also notice the circuit board has one less pad than required because I envisioned a potentiometer to be able to tweak the gain for different guitars. In practice this was not a problem because I just twisted the resistor and wire together there, and I only have one guitar, and if I make it tweakable then the temptation is there to get another guitar, which totally goes against the initial paragraph of my 'ible!!!