Place the porthole frame on the bar where you want to install the porthole. Trace the outline of the porthole frame with a grease pencil. Remove the porthole frame from the bar.
Drill a hole in the bar just inside the outline with a 1/2-inch drill bit. This is a starter hole for the jigsaw blade, so make it as close to the outline as possible.
Cut out the hole in the bar with a jigsaw just inside the outline marks. Place the porthole frame inside the hole for a test fit. Clean up the edges of the hole with the jigsaw if the porthole frame doesn't fit.
Remove the porthole frame. Place a heavy line of silicone sealant around the edge of the hole, on the top of the bar where the porthole frame lip will rest. Replace the porthole frame and press down firmly.
Wipe the edges of the porthole frame with a damp rag to remove any silicone sealant that squeezes out.
Install the porthole in the porthole frame as per the manufacturer's instructions.