I actually found a way to do both. I use the elbow joints on my PVC pipes. As long as they are 1/2 the length of my gar they don't even try and get into the pipe. I use this technique so my eels have a "safe spot" if ever the gars do start getting aggressive towards them ( which knock on wood hasn't been so) Been doing this almost 2 yrs now and it works really well. Just make sure the elbow joints are secure and won't fall off. I had one fall off once and luckily caught it before anything bad could happen, and my gar was chilling out inside it when I shooed him out.
But x2 on be very careful w/ any decor that could trap and drown them.
duck tape alone on your top to hold it in place wont work. you need rocks or some other form of weight on the top as well. my large florida has been on the floor 4 or 5 times to prove this.