Don---Here's a neat trick. Figure out what diameter of hole you need, and what height the "bubble" has to be. Go to the cooking ware shop and get a pair of "tupperware" type plastic bowls that are the correct diameter and depth. Take a peice of heavy cardboard and cut a hole in the center, just big enough to let the bowl slide through, then tape it to the bowl from the back side all around so that the cardboard is flush with the open side of the bowl. Lay on a liberal coating of pastewax inside the bowl and about 1" to 2" onto the cardboard. Lay up 2 or 3 layers of mat and resin on the inside of the bowl and extending out onto the cardboard.
When it hardens, pop the glass out of the bowl and off the cardboard, then trim around the flat area that extended out onto the cardboard so that you have a bowl shape with a flat flange about 3/4" wide all around it. Cut the holes in your floor and stick the bowl up through the floor from underneath. Then you can do what bonding you require between the flange and the underside of your floor.----the bowl shape won't need any bodywork on the side that shows, as it will have conformed exactly to the interior shape of the bowl.