Decade variable resistor 0 - 9,9Mohm* in increments of 1ohm with a 3d printed enclosure using moderately cheap and easily sourceable components.

Total cost for the parts orders looks to be just under 20€ The soldering is rather tightly packed, so proceed at your own risk.

A note about resistor tolerances: I'm using 1% resistors. I wanted something better than the really cheap and common 5% resistors, but much better than 1% costs too much. Also, the switches themselves add resistance and some variance. The tolerances don't add like some people worry for setups like this - any resistance I can dial in will be within 1% of the number on the switches - in theory. The switches add a couple ohms (1.5ish) to the box overall, so the low values will be more imperfect than the high values. Also, it's best practice that once you tune a circuit with a box like this, you remove the box then measure the resistance instead of relying on the readout.

Parts:

10-digit decimal switches x7
Banana plug connector x2
1% resistors 1/4W 9 each of 1, 10, 100, 1k, 10k, 100k, 1M

There is a simplified drawing of the switch with only 6 contacts drawn. You would measure the resistance between the common and pin 0. In this case the wiper is switched to 3 so the measurement would be 3xR. You can daisy chain these up to provide more resistance, just attach the common to pin 0 of the next decade.