Be patient and search ebay every few weeks for "rich lean" and you can score a simple rich/lean indicator for $25 like I did. It gives me great feedback on my Holley Pro-jection since I changed to the 2D box that doesn't have the O2 light of the older box. If you want to roll-your-own with just a digital multimeter on a narrow-band sensor, you could make an R-C circuit to filter the fluctuations. If so, use a ceramic capacitor, not electrolytic or you may ruin the O2 sensor. Similarly, don't use an old voltmeter with needle (galvonometer) since they draw considerable current which can damage the sensor. At least that was the story when they first came out in the 80's. Even it it doesn't destroy it, you would draw the signal down. You can find simple universal O2 sensors cheap on ebay. All you need is the 1-wire type. My 82 Aries had that if you want to try rockauto.
You want cheap, I am Mr. Cheap (see tagline). I got an O2 sensor w/ indicator LED and exhaust bung for ~$5 brand-new on ebay. It was called "emissions warning light", for Onan generators I recall. Used it on my car ~6 mo. to monitor my Pro-jection but has ~30 sec filter so the delay was too long to grab the Pro-jection knobs before the engine stumbed (lean), so I changed to the Holley indicator. For a carb, wouldn't matter since no knobs to tweak. I got a bunch of new stuff cheap from the same surplus house, including an MSD wide-band O2 sensor. If in a rush, PM me and I will sell what I had on my car (or equiv.) for $15 + shipping. Just a rich/lean LED, not wideband.