Improvement suggestions on monster trap in Minecraft?

I have built a monster trap in Minecraft that has only one problem.

If I want to kill monsters, it has happened quite often that a skeleton shot at a creeper that then exploded. Recently a creeper tore half of my way down. Do you have any suggestions on how I can prevent this?

Collect magma blocks for damage and funnel for lood

Just drop the mobs into a 2x2 slot and that shouldn't happen anymore. Including a few hoppers (funnels) with slabs (half-steps) to have the loot picked up directly.