What model/year Isuzu do you have?

I did a somewhat similar modification to my intake. I dremeled out the front of my airbox, then removed the sheathing around the headlight to allow cold air to enter the airbox directly. This in combination with my dynomax muffler gives a better throttle response and more power. This has really worked out better than the calmini cone filter for me. However, I did not notice any increase in MPG. In fact, it might have dropped a bit, but that's probably due to 'more spirited' driving egged on by the new vroom-vroom sound my truck makes.

I plugged up the air inlet hole that you are using as your primary inlet on the side of the airbox since it is facing the engine and is sure to be exposed to hotter air (even with a heatshield).

Wish I could get 18MPG city, with gas prices around 2.30 in my area.