I may have asked about something similar a long while back, but I'm
getting closer to doing it now.

I need to build a one-piece "top" for some off the shelf bookshelf
units. The top will have two functions: to make the three units appear
more like one integrated piece of furniture, and to provide a durable
sun- and water- proof surface to cover the damaged tops of the units.

The three units together are about 9' long and about 30" tall. They used
to sit under a long south-facing window. We intend to move them back
there. In addition to the fading and other sun damage to the tops, there
were a few spills over the years; we had some potted plants there.

I intend to use some sort of tiles as the "field" surface; big ones, 20"
wide probably. They will sit in a "tray", made of 3/4" ply with a 1x2
solid wood border on all four sides. The top of the 1x2 will either be
flush with the top of the tile, or perhaps a hair lower. I figure I'll
use pocket screws to fasten the 1x2 to the ply.

So, here are my questions:

Should I use ply? Or would MDF work just as well? None of it will be
seen, and it will rest on the tops of the cabinets, so it won't hold any
weight.

What should I use to fasten the tiles to the ply? My original thought
was "nothing". Gravity and the "border" could hold them in. But I wonder
if gluing them down somehow would make them less prone to cracking
should something drop on them. Of course, if they're not glued, they'd
be easy to replace.