Let it go, man.
Allow me to clarify and describe a common, effective and relatively cost effective way to secure windows and reflects how most windows are secured in vacant buildings downtown.
If you want to minimize further water infiltration from the window openings, the plywood should be exterior to the window and covering as much of the opening as possible, affixed to the window frame if feasible. In this case wood blocking is attaced to the window frame to bring anchor points nearly flich with the exterior. The plywood is then attached to the blocking and pulled as tight as feasible to the building exterior. This prevents glass from falling outward and directs most water away from the frame, where it can penetrate the building evnevlope and create spalling and other exterior degradation issues. It doesn't solve it completely, but it stops the vast amhority of water penetration that occurs at the windo openings.
If the frame is too far degraded to hold the plywood, as was the case for most of the windows at the Metropolitan, a system is created in which 2x4 or 2x6 crossmembers are nailed or screwed in place across the openings. These crossmembers are interior to the building and anchoed to the interior walls. Wood blocking is attached to the crossmembers to bring it close to being flush with the exterior. Bolts are passed through the plywood, the blocking and the bracing beams and tightened down. . This draws the plywood flat over the opening and secures it against the winds that can occur during blizzard and near tornado conditions.