Mixing in mono

I use a single mono Auratone type speaker myself. And a single speaker is still better than two small speakers as well. I believe it is best to mix the stereo buss outside rather than inserting plug-ins and things and trying to sum for mono that way. By mixing outside you never have to think about it again, it is done.

I use a spare headphone output on my mixer and have a small Teac passive mixer connected there and then feed a power amp and into the speaker. You could also use the headphone jack and just use two series resistors eg 100 ohms connected to the ring and the tip and connected at the the other end. That way you wont be loading down the output of one channel with the low output impedance of the other which may create distortion. Gain is usually easy to pick up either in the power amp or the active speaker itself.

You could also use a small active line mixer as well if you have something lying around you are not using.

A small mono speaker is amazing not just for checking mono compatibility but also for doing complete mixes on. It will improve your mixes a lot. It is good for a great many aspects of mixing.