Here's a couple pics of the larger cave that I built. This was a case of eyes bigger than tank. I had it all mapped out in my head, drew up plans, cut and glued the plexiglass, painted it, then put it in the tank. Too big
I forgot to take into account the amount of space for the rocks. This original box only left me with about 2" of foreground space, so back to the drawing board. The new box is only 6" instead of 12" deep, so even with the rocks I still have about 8" of foreground space.
Here are pics of the original box. The second box was built exactly the same, just smaller. I realized I should have taken pics along the way after I built the second box so there won't be any step by step pics to follow.
The space in the top is approx 3" tall for substrate. I plan on putting in a lot of Crypts so I wanted it to be deep enough for their roots.
This is the bottom. All the cutouts will provide the eel plenty of options for going in and out. I used PVC glue to attach the plexiglass dowels to the midsection and then glued the walls. I went ahead and covered it with a layer of silicone for extra support just in case. I used my son's Yu-Gi-Oh card tin to hold the midsection level and in place while I attached the walls, one at a time.
Just a close up of the dowels
I was a little concerned about the weight of all the sand on top putting pressure on the glue. My unique PVC plumbing goes under the sand and then I used a 90 degree elbow with about a 1" piece of pipe and a cap under the cave. The roof of the cave sits just barely above the 2 caps so if the glue does fail the roof will land on the pipe and not collapse.