Oh dear. Well, pretending to be bound might be good for you in the moment to keep the energy moving forward but I hope you let them know, gently, afterwards so they can work on their skills. Better yet, you should ask them out to some rope classes in your area some time. I know a number of great bondage tops who got their start learning the ropes by hitting up some of those local workshops with someone they session with or met at a munch.
Now, as to immobility...
To be honest, I would never assume saran and duct tape would provide immobility on its own. It's a lot of fun because
one can move. Making someone squirm about the dungeon floor is the purpose of that kind of mummification for me. Love to see those bondage bottoms wiggle across the floor to try to kiss a toe... or worse.
I am a fan of playing around with immobility. I suppose it's a hobby.
I designed (and need to re-design now that I've had it for a couple of years) a kind of flat, padded stokes litter in steel for suspended bondage that allows me to play with variations on mobility.
Total immobility is a challenge when it comes to fingers and toes, I can agree with that. I've done it, not easily, but it's possible. Yoga Toes
But in terms of fully fixing the human body in place? I've made it incredibly easy to do this with a willing victim in my dungeon. The right tools for the job, for me, involve the 'stokes', leather straps, and a few pieces of high quality bondage gear. I'm not a rope lover.
Ratchet straps and a body bag are even easier, of course. I always chuckle a little when I put a strap around the feet as the final piece with a sleep sack.
I do think immobility can be a challenge. You might be surprised by how incredibly hard it can be for the bondage puppet to endure, particularly with the neck and head fastened.