Hmmm, I would say let them push something through as well, like a knife or something. Also, don't jump the gun on saying "I know what you're thinking." Say something like "Isn't that amazing!" and they'll just be like "uhhh, isn't the box empty/all the cards have slits?" Then you reveal the block of steel. Maybe do a snap also, since snapping is the universal sign for something magical taking place.
The way I would open up would be to say "Ok, the funniest thing happened at my parent's party the other night. I was doing magic and I asked some guy (who had a little to much to drink) cut the cards. So check this out (take out box) he took a knife and actually CUT all the cards! It was so funny I had to keep the box, here you try. Isn't that awesome? Simply impossible, right?" Then do the reveal.
Also, just keep on performing it and performing it, try different approaches, and the more to you present it the more you'll see what works and what doesn't. Wayne Houchin said it took him about 2 years of performing Sinful to finally realize that some things just weren't working out. Just give it time. Performing it is the best way to find out what works.
Best of luck! Hope I helped out in some way.