This end cap is designed specifically for Penn Elcom's R1196 series heatsink project boxes.
20-300 - Silicone Heat Sink Compound
Used for heat transfer from semiconductor devices to mounting points White Will not dry or harden 1 oz. tube
I have been dealing with MCM for over 30 years now. If I didn't like the service I'd would have found a different supplier 30 years ago. For most of my needs they are the 1st stop on my shopping list.