I almost always put a zipper opening in my custom slipcovers for easy on and off.
For most styles, I insert the zipper in the left back corner. My customers don’t know it’s there until I point it out.
You might think it’s an invisible zipper. It’s not. It’s just a regular #3 nylon coil type and I sew it in much like you would a lap zipper.
But the trick to making it look invisible is how you close it. Here’s how: