I once did some work for a HVAC contractor during the construction of SCI facilities, I did not require a security clearance because they were unfinished and unoccupied.
I owned a small metal fabrication company at the time, the first requirement was that ALL heating and AC duct work have welded steel grates inside them with the specification of Minimum 1/2" cross section round or square steel bars welded into a frame the size of the duct interior and screwed to it with no opening more then 1" in size.
This included 5" X 5" returns, I made hundreds of these over a period of years. They came to me one day and said "we have a problem", they had to add all of the security features to an existing commercial building in order to make it SCIF for a weapons contractor. This was apparently a very high level of SCIF, the above mentioned grates were to be fastened with steel threaded rods passed through the duct and grate frames in 2 positions 90 Deg. apart, double nutted on each end then the top nut welded on each end. I told the contractor that I did not do on sight welding nor did I have the equipment to do so.
He said you are going to have to do it, I told him that I had no idea how much to quote for such work and his answer was "as much as you want", can't beat that so I rented a mig welder from HD and knocked it out in 3 hours or so..
One of the other SCIF requirements for duct work is the installation of fabric bellows in the sheet metal that stops the sound of voices in a secure room from being conducted outside of the room via vibration in the steel.
Good work if you can get it.