@Decuchi23 - There really doesnt need to be a "Box" per say, just something that can support the weight of the speaker in order to not place all of the speakers weight on to the plastic cubby in the floor, and something to keep water, dirt and dust from damaging the speaker itself.
I plan on using a 1/4" thick plate of aluminum that I will weld or bend at the edges to create a 6-8" deep 5 sized "box". I will cut a hole in the square end to fit the speaker, and will also cut a hole into the floor cubby the same size. I will then sandwich a high density foam between the cubby and plate to make it weather tight. The aluminum "box" will then be supporting itself and the speaker by attaching it to the bottom of the truck (likely with welding tabs onto the floor plan so that I can use isolation bolts to keep unneeded vibrations from transferring to the cab). On the open end of the "Box" (where the magnet of the speaker will be), I will use two pieces of expanded metal to sandwich a hydrophobic membrane, polyester filter (ac duct filter), and natural wool fiber sheet to create a low resistance filter to keep moisture, dirt and dust away from the back of the speaker. This filter will be removeable and will likely need to be cleaned every so often as well, and this way it is serviceable.
If you ever have the chance to hear a true IB (infinite baffle) sub stage in a car, I strongly suggest you take the time out of your day and listen to it... they are very accurate, dig very deep into the 20 Hz range, and have a sound all of their own. Its not for everyone though, as you normally need 2-3x the speaker surface area to get the same SPL as a sealed or ported speaker. Oh, but you only need a fraction of the power compared to an enclosed speaker, because there is nothing "cushioning" or limiting the speakers ability to move.
Hope this helped explain how I plan to go about this.