Custom box, too much airspace!

Alex, remove all the pieces of mdf from the inside of the box. Next get some packing peanuts. Find a large container like a bucket of some type and fill it with 28.32 Liters of water. Mark a line on the bucket where the top of the water is. Empty the bucket. What you have now is a bucket of known volume. 28.32 L of water = 1 cubic foot of airspace. Fill the bucket as many times as needed and empty into the box to calculate the box internal volume.

To take up space inside of your box and to keep things light, try the following. Now that you know how much volume you have and how much you need to take away, take some styrofoam insulation and cut it into pieces to take up space inside of the box. Once you have the pieces cut and stacked into whatever shape you are going to make them, use duct tape and seal the styrofoam inside. This if wrapped properly should make it airtight and nice and light. Attach to the box by hot glue, spray glue, resin etc. to ensure it does not rattle around.