I made a video to show how to split an item into multiple collision boxes, letting you walk through gaps etc. My demonstration uses an arch I initially added but of course couldn’t walk through, till now.
It really is as simple as taking your final product and hitting Y to split the mesh and P > Selection, to make a new object, which in turn makes a new collision box.
Hope this helps someone!
Also, I’ve seen other models made that you can enter but they are extremely laggy, I’d recommend keeping it to a minimum.