I found a DIY for the Corsa C from the UK froum. Here we go:
Note: This doesn't serve any sort of performance difference, but it does make a little more noise, better than putting a cone filter in in my opinion. I wasn't comfortable with drilling massive holes in my airbox, so this had to do.
Tools required for this is 1 phillips screwdriver, 1 pair of pliers, a drill or dremel, and some patience.
I assume you'll know how to open up your airbox, if you don't, close the bonnet and don't attempt this.
Once you've opened up the air box. You have this under the panel filter.
This is how to de-restrict the air box at the bottom. For this you can use a dremel or drill to get the plastic rivets off and end up with this result.
Once done with the bottom part, you find that the top part has this cone thing which needs to come out. It looks like this:
This cone thing can be cut off using a dremel or with lots of brute-force and swearing, it pushes out. If you have bad blood pressure, cut it rather.
Put your airbox back together, not forgetting to put the filter back in like I did
and test it out 
