If I have a gravel vaccuum, can I separate the head/tube part, put a sponge on the bottom of it and leave it in, attaching the tubing when I need to siphon? Rather than, say, finding a PVC pipe that the tube could fit inside?

And a second question: If I do either of those things I'd need to cut a hole in the net to let the tube reach the drainage layer. Can you recommend any ways to keep the substrate from working its way through the hole into the drainage? I know a pipe will be there, but things can shift around.