I would think if you had it several inches in the pipe that it would be completely bathed in the exhaust & no outside atmosphere there but evidently that ain't the case that the probe needs to be way up in there for an accurate reading.
By the time the exhaust gas gets to the tail pipe, it's lost a lot of heat and pressure and starts to mix with ambient air. Those factors will alter the composition of the gas enough to change the O2 reading. Most newer cars have the first O2 sensor in the head pipe right after the manifold. 4 cylinder cars have the sensor where the tubes merge coming out of the head. The downstream O2 is after the cat as a comparison to the front to make sure the cat is working and are sometimes calibrated differently. The sniffer was generally used for pre-OBD emissions tests where the car was driven on a dyno under different loads. Once the O2s came out, the sniffer became obsolete.
If welding a bung is not in the cards, why not get an O2 sensor clamp kit? Drill a hole, wrap the clamp around the pipe with the gasket inside, tighten it up and you're good to go. I used one of those on my truck with an Autometer A/F gauge. Worked good.
