Hi, Stanley. I can't help you with this specific application but I did just do a mosaic tray with a very complex shape, the Detroit Red Wings logo. When I had all the parts in about the right place, I just ran masking tape over the top. I was able to pick up the whole tray section, about 12 x 16" and install it all at one time. I did have to make final adjustments but this also served to set the individual parts in the mastic.
You might think about building a jig for your tiles and using tape over the top.