I don't recall seeing any builds on the forum with a built in drip pan. It's certainly possible, just keep it low enough that it doesn't affect the air flow.
How much gap do you think from the firebox to the drip pan. Do you think there should also be a gap at the smoke stack to allow for air to flow under the drip pan? Im assuming that not all the air will go up and over.
I used to put old cookie sheets on the bottom of my old offset to catch the drippings. They just sat on the bottom of the cook chamber and made cleanup a bit easier. It almost sounds like you want to suspend them. It's possible but keep in mind they will then act like tuning plates and mess up the heat distribution of your pit.
Im fairly new to this, so is a tuning plate a bad thing? By mess up the heat distribution, im assuming you mean it will change the flow of air in the pit. My thought process on this is that it would help keep the pit cleaner by catching and diverting the grease to a drain pipe, instead of using the bottom of the pit to catch the grease and then to the drain pipe.
A tuning plate is not bad. It is often designed as a series of plates with gaps which allow the heat to evenly run across the cook chamber. You should like you are describing a baffle plate which is a solid plate that traps the heat under it in order to release it on the opposite side of the pit. They are only used in reverse flow pits tho. Since you have a standard offset, your best option is to use the bottom of the cook chamber.
Maybe send a diagram of your thoughts, I could be making wrong assumptions.