On some recommendations from the helpful folks on the Fat Dragon forums, I built my second tile by cutting outside the lines for the column holes. It turned out to be about 1/32" extra on each side, so the hole is 1 1/16" on each side. The column certainly fits into that, but it's a little *too* easy to fit it in now. I've got 9 more tiles printed and waiting to cut out, so hopefully by the time I'm done, I'll be a pro at getting the cuts just right. I'm thinking I'll cut them 1/32" extra on only two of the four sides next, and see how that goes.
I'm considering putting a layer of cheap cardstock or plain paper on the underside of the tile, to keep the pillars in place. I'm imagining making a 2nd floor to some things, and I'd like to be able to make it just like a 1st floor.
The use of TLX-style tabs on the sides seems to work well. I'm considering making railings that plug into the sides like TLX railings, and it would work to hold a 2nd floor in place.