An O2 eliminator is just a plug containing two resistors. One is to provide a load to simulate the heater element so the ECU doesn't throw an error, and the other is to stop the ECU receiving spurious signals from the missing O2 sensor itself. An O2 sensor isn't resistive, it's a voltage device that creates a voltage based on the amount of O2 present. The 3K resistor simply pulls the signal to zero volts, so the ECU thinks the bike is always running lean, or in most cases the ECU considers the O2 sensor to have not yet heated up and stays in open-loop mode.
Is it a good idea on a bike with no programmed ECU? Well not really. The O2 sensor does a good job of keeping the mixture at 14.7:1 when cruising or idling, which is ideal for efficiency and not building up carbon deposits. Under acceleration the O2 sensor is ignored anyway (open loop mode) so it's not relevant.
Why have them at all? Well if you have a PCV and it corrects the mixture to your desired 13:1, with an O2 sensor the ECU will try to lean out the mixture again, so you disable the O2 sensor so the ECU doesn't detect anything amiss and doesn't 'fiddle' with the fuelling.
Re the booster plug, if you really want to try the effects on a 25°C day just buy a 2K resistor and stick it in series with the pink wire to the airbox temp sensor.
If you really want to copy a BoosterPlug and get the huge benefits (cough) of a temperature independent fuel increase, buy a 2.2K NTC thermistor and a 56K resistor. Wire them in parallel, then insert them in series with the pink wire. You don't even need a PCB, the circuit is ridiculously simple.
Here's a pack of 10 NTC thermistors for $4 : https://www.ebay.com.au/itm/10pcs-23...Y/263450775678
Here's 100 56K resistors for $7: https://www.ebay.com.au/itm/Resistor...s/123709340940
So you can build your own BoosterPlugs for under 50c each.