Only one speaker

NO never.

Do you have a mono switch on your preamp? It's fairly uncommon these days but some still have them.

If you don't have a mono switch, you will, at a minimum, need a Y connector that combines the left and right signal into a single, summed, signal to feed the single amplifier (or channel) that will be powering your one speaker.  

I have heard a number of mono systems, and using a transformer to sum the two channels sounds better than a simple Y connector.  Of course it is a more costly approach that I would recommend for someone doing a permanent mono system and not something that is just temporary fix.  A local dealer in my area has a tech who has built a comparison device that allows one to switch between Y connector and various choices of transformers so one can hear what each transformer does to the sound.  I am not that big into mono, but, I was surprise at how good a mono system can sound, particularly when care is taken to optimize the sound, including using the right transformer for the job.

Yes, the Y adapter sounds like the way to go if the workmen dawdle. My point, though, was that hearing just the one channel wasn't awful at all, so if I have an adapter laying around, I'll do it. Otherwise...

This article should answer your question! Nothing to do with mono!
https://www.audiosciencereview.com/forum/index.php?threads/crosstalk-channel-separation-how-much-is-...