How do I make a one-way trapdoor?

I'm working on a mob trap like the one shown here, but the slime just jumps out. I have decided to make a one-way trapdoor so that monsters can fall down into it, but it can't jump out. Please note I do not want farming techniques, I know this isn't the best, I just want to know how to make a one-way vertical gate.

I haven't used any exact sciences, but from what I've seen, if you try using slanted blocks and funnel into your hole, the slime will in fact jump in at some points (slime's contact hit box is just under 1x1). This also solves jumping out. Because the slimes jump at an angle, they will have no ability to jump back out, so long as the hole isn't along a wall.

Try something like this:

-\_/\_/\_/\_/-

- is solid / is downward left slant \ is downward right slant _ is open air