JBFan:
A few similar situations here.....
We had a 2' x 2' alum panel fabricated to fit in the ceiling grid. We punched the panel to accept twistlock devices. On the "backside" an 18 x 18 screw cover box, with the bottom 16 x 16 cut-out. Greenfield (6') into the box with whatever circuits were required.
THe plate was clipped to the grid tee's, and support provided by threaeded rod to the z bars.
The above is in a vitamin mfg plant, and yes, it is "movable" to an extent.
The line equipment is relocated to accomodate whatever products they are running, and we have only relocated two panels since it was installed.
On a smaller scale the t-bar box should work fine for you, you may consider an additional means of support also.
John
