How to stop creepers blowing up my Zombie XP farm

I have a zombie spawner that I have turned into an XP farm.

The problem that I have is that creepers spawn in the zombie spawning area, and end up at the bottom of the 22 block glass chute. Then, because there are a lot of zombies I don't realise there is a creeper, and it blows me, and the chute, up.

How can I prevent this?

Whole design:

Close up of changes:

Here you end your chute with a half slab on a hopper and a half slab at eye level (2 the underneath of the block 2 higher than the hopper. Because the creeper cannot track to you, even when you hit it it will not blow up. To be safe, I wear a golden chestplate with blast protection (that I got from the zombies) and I build the end of the chute out of Obsidian.

Here is an image of my solution in MC:

You can see the half slabs (cobble) and the chute (glass, at the top - it doesn't join because of the creepers and lack of sand near me). Under the base slab is the hopper with chest: