If you want to make a really slow trap that's almost impossible to escape make the floor ice and lay soul sand on top then put cobwebs on top of the soul sand and to make it even more impossible to escape put a 3 by 3 square of pressure plates in the middle with pistons facing up next to the pressure plates like this

And if they manage to escape they will have the ice, soul sand and cobwebs on the outside as well but they should not be able to escape any way just like one I've made before.