Question about shapes

Okay guys, I've been struggling with this for months and can't figure out how to do it. Here's what I want to do:

I've got a model moving forward in 3d space, and behind the model, I need to create a wall. But instead of the wall moving forward with the model, I need it to create a new wall every 10 or 20 pixels that the model moves forward. Does that make sense?

I can make the wall no problem, but I can't figure out how to get it to create a new wall when the model goes so far forward in space. Does anyone have any ideas?

One idea, keep the last position you created a wall in, stored in a vector. Then, every frame, check the distance between the current position and the stored one. If it's larger than the distance you want, then create your wall between those positions.