I bought those fixtures from Homedepot too. I think it is called shoplight in fact, and only costed $9 or so for 4' unit. Then I took them apart, and used ballasts and sockets only. It is not difficult to take them apart and not that difficult to re-wire them either. The finish is nicer since you do not have to work with the housing. I have a lot of housing sitting in my studio by the way.
I made the box with plywood, but this time the unit is a stand alone type with rollers at the bottom. We use the bottom space for storage space. We can roll this around wherever in the space, and we do not need another flat surface to place it, and it creates a table level flat surface for us.
We needed a larger one since we use it for workshop and class setting. the lit area measures 48"x30". We just made 48"x38" one for a photographer in NY. That was pretty big. We used all the same method.
By the way, I do have a several of spare 24" BL bulbs if anyone is interested.