You could get a stereo to mono summing box, like this one.

https://www.amazon.com/OSD-Audio-SMC21-Converter-Isolator/dp/B07FFBJ8KR

But the $35 you'd pay for that thing already puts you more than a quarter of the way there to getting a second E5 XT.

The music on your records is meant to be played in stereo. Save up for a little while longer so you can play it in stereo. If your budget was limited when you bought this thing originally, you should not have bought only one speaker. You should have bought two cheaper speakers.