Thank you for your inquiry on the CP053. Regarding us mentioning "external 5.25 bay", we mention that bay in that manner to differentiate between internal bays that are not easily accessible, such as "internal 3.5 bay" compare to "external 3.5 bay" which means the floppy bay that is 3.5" size. If the product is completely external where you connect via some sort of external port connection, we would generally not make the product fit inside a computer case and would not make it to those standard sizes.
As for support for aftermarket heatsinks, it would be depend on if the aftermarket heatsink fits into the plastic bracket around the heatsink, as that bracket pushes down on the heatsink and helps keep the m.2 drive secure.
If you have any questions on our products or this concept product, feel free to send us an email or message on the forum.