A few lengths of simple under-substrate cave that your fish can explore and you can watch them in since this goes against the front glass. Substrates and plants can be put all around it and in the center hollow. Just keep the tunnel itself clear. I have a 5",7", and 9" long version and the 5 has supports to help keep it upright since the surface area is smaller and less sturdy. Additionally, these models can be scaled for almost any size you need in your slicer.

I suggest printing it on its back face and using supports for the support legs. I also recommend sanding any rough spots to protect your delicate fish friends and their fins.

Also, this is about 2.5 inches from top to bottom so it would work great for you so long as your substrate is at least 2 inches tall in that area.

If anyone does use this in their tank please post a make and show me. I'd genuinely love to see it.