My Mac Mini has no slot for an SSD. What are my options?

I have a Late 2014 non-fusion drive Mac Mini that I had planned on converting to a fusion drive by installing a 128 GB SSD into what I assumed would be an empty slot and pair it with the stock mechanical drive. Unfortunately, when I removed the cover I found that there is not slot on mine. I’m assuming the non-fusion drive models did not come with that port populated? Is there an economical way for me to add a flash drive to this model or is my only option to pretty much just replace the stock mechanical drive with a 2.5” SSD?