After all of the necessary holes are drilled in both PVC caps, it's now time to glue them together. It's best to sand down the edge of the inner cap in order to make a larger and flatter surface area than you would normally have with it's rounded edge. Standard PVC primer and cement procedures apply here. Apply PVC primer to both surfaces in order to soften them up and let dry for a minute, Then apply PVC cement to both surfaces and put them together. Position the inner 1.5" cap so it is centered and leave both to sit and cure for at least an hour.