I've finished the structure that will receive the tile now. I replaced the plywood doors with maple veneered plywood since it was a much straighter/flatter product. Shame to tile over it as it's a nice board but oh well.
The doors have a lip screwed and glued onto their tops and ends. The sides of the stand were built-out 3/4" to flush with the new door lips and these ends also now have a lip on top so the tile strip for the stand's top is level around the 3 sides.
When the doors are shut the stand is a perfectly flush box so hopefully the tile plays nice and will do the same. The plywood trim for the tile is about 1 cm away from the foam's edge so it won't interfere. I'll tile once the tank is on the stand and filled so I can tile close to the tank's glass with all deflections observable. I had tiled around my 400 gallon before filling it and it bowed so the glass touched tile in a few spots which was scary...
Next I'll be removing the doors and painting their insides.
Here's some pics:
