Not sure I follow what you're actually doing, but if it's not in the same plane, the panel will look awful.

You might consider installing some wood backing so you can screw in a wood panel and then cover the gap with some sort of trim. Or you could add a vinyl "T" edge to the wood panel itself, which will cover the gap when it's screwed in place.

Your backing could be a couple 2x4's which bridge the gap resting on the adjacent drywall. You would would screw in place through the adjacent drywall and mud over the screw holes. Then you could use that for one of the solutions above.

Just one way of looking at it. Good luck.

BTW, why not make a smaller hole and use one of the plastic spring-loaded access panels that you can get at Home Depot? They have a 9x9 and a 13x13 as I recall (caution: you need to cut the hole smaller then you might think, so buy one first and figure out how it works). Also, they're not sold in the same section as the steel access panels, so you have to look around and/or ask multiple people in the store. (They never seem to know they sell them, but they do.)

Thanks for the reply. I'll go look for those access panels, haven't been able to find them at a local store yet. I think I'd prefer the larger hole, though, it would enable me (or someone else) to basically stick my head up into the space between floor joists and suspended drywall ceiling and work there -- all the electrical cables are coming up from the panel at the wall above, and with a small hole it'd be difficult to have access to them all (and I bet I'd need to access the one I can't reach later...) Also, I think having one large panel actually may look better, as it seems more like intentional design rather than "oh, someone had to cover something up there...".

Actually, after looking more closely at some ceiling tile, there is some which has recessed edges, so that it'll stick out under the grid. Which while generally better looking wouldn't work well in my application. But other panels are just flush, so they'd at least maintain the level of the ceiling. Then again, those I've seen have really rather ugly texture. And then I realized, I could just use the T-rails from a suspended ceiling system, and put in a piece of drywall instead. It's heavier, but I think it would work, and then there are no issues about mismatching texture/color.

The other alternative would be to use one of those more fancy tiles, to intentionally create a point of contrast. Like a silver-metallic grid.

Decisions, decisions...

Why not just use another piece of drywall instead of a ceiling tile? That way, the only thing you'll see is the frame supporting it. If everything is painted the same color, it should blend right in.

Any drywall supply house will have various sizes and styles of access panels. Much better idea than making your own which will look out of place. 2x4 seems quite large. 1x1 should be fine, centered in front of the panel. I think HD has plastic 1x1's.

Using a "real" access panel will look much better. Next time you're in a hotel room, for instance, take a close look and you'll probably find several.. but wouldn't notice them unless you were looking.