Heatsink isn't going to help much if the panel is fully closed off.
Is there enough space to run an m.2 extension ribbon from there to the main chamber?
Are you opposed to drilling into the case panel above that slot to open it up to some fresh...
The question is why? It's not the latest version of DOS. It's not even 6.22 which is popular. There's been free versions like FreeDOS, which is the preferred way to flash a bios. Windows doesn't even use DOS.