Tom:
IF you have a rated ceiling assembly, then you must provide 'equal' rating with the devices you install.
First, check with the local AHJ & determine IF the drop ceiling is in fact a rated assembly. The mfg install instructions will be a big help in this also.
IF you have to maintain the rating, the Tenmat covers are the easiest approved for maintaining the rating.
Some 'old school' metgods were placing a 'tent' of 5/8" fire code sheetrock over the top & sides of a troffer. Another was laying 'rotten cotton' insulation over the fixture. Some AHJs accepted the described ways, others did not. The Tenmat is UL labeled/listed for troffers and rated ceilings. There may be other brands, but that is the only one I am aware of.
