Store bought floater frames come with brackets that you screw onto your stretcher bar...as you tighten down the screw it pulls the stretcher bar tight against the floater frame and locks it in.
This is a store bought floater frame, (black and grey area), showing the bracket screwed into the stretcher bar.
On the frames I made all I did was pre drill a small hole through the floater frame and into the stretcher bar. This keeps anything from cracking. Then you just screw into the stretcher ba to lock it down. I had some scrap pieces of foam core which I slide into the gap between the painting and the floater frame to make it level in front before drilling anything in back.
Get someone to help you do this...they can make sure the painting stays level and hold the painting in place while you drill your screw into the back of the stretcher bar. I used sheet metal screws because they don't have that taper behind the head like most wood screws...sheet metal screws are flat back there. Otherwise you need to countersink your screw holes.
The grey bumpers were screwed on with shorter screws and keep the bottom of the frame from scraping the walls and make the painting hang level against the wall.
Way to go on the gallery...that's excellent John!