Measure the width of your PVC pipe and the thickness of your plywood. Add these amounts and cut a piece of plywood for the back at that width.
Cut pieces of plywood to fit for top and bottom. Attach the top piece.
Slip PVC tubes into the box (Image 1). Add the bottom piece of plywood (Image 2). Secure PVC tubes by screwing through the back piece using 1/2” screws.