Step 3: Cut the holding plate
The lamp holders have two plastic sleeves designed to hold a lamp shade. However, we can make use of them to fit the bulbs into a plate which will then be mounted into a box. The terminal strip can also be mounted onto this plate.
Decide how big your box will be. Mine was about 16" wide, 13" long (along the length of the bulbs) and 4" high. This gave me enough room to fit four bulbs with adequate spacing, plus the terminal strip, with space in front of the holding plate for the bulbs themselves, and space behind (about 4") for the wiring.
Cut your holding plate from the MDF. It will need to be thin enough to allow the sleeves of the lamp holders to screw down securely. (I used 1/8" MDF).
Mark out on the MDF where your lamp holders and terminal strip will go. I allowed 3" space between the central points of each bulb, giving about 1½" space between each bulb when fitted. [See picture 1]
Drill holes for the lamp holders using a hole saw or spade bit. Better to create holes a little on the small side and then open them up with sand paper than drill them too large. You want a snug fit for the lamp holders. [See picture 2]
My MDF was white on one side. If yours is just plain uncoated MDF, now would be a good time to paint one face of it white to make it more reflective.