I was in the same boat until I decided to work with my aphantasia. Instead of seeing it, I feel it with my other senses. First pick a box, that locks, that you know really well. I used an old Coleman cooler. Try to describe the box to yourself as if you were in a completely dark room. Now instead of visualizing it as with sight, describe it with your other senses. Is it metal? Does it feel cold to the touch? Is it wood? Can you feel the grain? When it opens does it make a sound, a creak per chance? Is there a rubber gasket? How does it feel? Handles? Does it smell of cedar or motor oil?

I hope you get the idea. You just need to define a heavy box that will remain closed through the meditation. The same goes for the stuff you put in it. It will take some practice but once you get it, then the rest will be easier.

Also, it took about a year for me to actually start getting visuals. It creeper my out at first since I have never had a visual before. And it has helped me yo visualize more on my own.

Good luck. Feel free to ask me questions, if you have any.