You can buy these ready made or have them made per your dimensions if you look around online or talk to your local framer. Since my framing budget is nowhere near that of many painters whose names you know I made this one myself...it is my second floater frame I've made and came out pretty good I think.
Here you can see the gap between frame and canvas. Also, the corners and edges. Notice the side nails are placed at bottom and top of frame so you can't see them. The back is nailed from behind so you can't see those nails either.
It looks like a ton of work but this second frame was much easier to build than the first...all of the figuring out was spent on that first one. I've seen versions of framing paintings where they built the frame flush with the edges of the painting and made no back at all. I've seen one like that made that was simply built of pine and then sprayed with black paint and it looked pretty cool too. Just a matter of what results you are looking for.
If you kept the edges all flat without the miter cuts and could find the wood in the correct thickness you could build this frame with just a miter box made of wood or plastic. It's just hard finding the wood in the right thickness. My wood is 3/8" thick and I started with 1" thick wood. That's why you need a table saw to do this. The table saw is also needed to cut the miter cuts. It's a fun project though....give it a try sometime.
UPDATE.....here is a pic of the back of the frame showing how the canvas mounts to the frame. The canvas lays flat against the back of the frame. Your stretcher bars of the canvas will butt up against the back of the frame. All you need to do is center the canvas in the frame and drill a pilot hole through the back of the frame and into your stretcher bar. Then simply put a screw in to hold your canvas to the back of the frame. I used 4 screws to mount this painting. To center the painting I use either pieces of foam core or bits of old mat board placed between the frame and the edge of the canvas...just add pieces until the painting is even on all 4 sides. ANOTHER UPDATE....Here are some additional pics of the assembly of the frame. Again, the shape of the frame is an "L" for each side of the frame. I did a mitre cut on each piece where they are glued together forming the "L". A mitre cut is a 45 degree bevel cut (actually any degree of bevel). For additonal strength I shot 3/4" brads, which are like finishing nails, in from the back of the frame....see diagram below.
Instead of brads you can use small finishing nails. I bought an air powered brad gun, which is a lot like an electric staple gun, at Harbor Freight for about $20. I already had a compressor to run it. I'm sure they sell manual or electric brad guns too....try Home Depot.
Here you can see from the behind the frame looking down from the top. The brads (circled in white) are shot from the top and from behind the frame. This keeps the viewer from seeing the brads when the frame hangs on the wall. Brads at the bottom of the frame are shot upward...the reverse of these tops ones. You can also see the mounting sheet metal screw going through the back of the frame and into the back of the stretcher bar holding the painting to the frame.
By shooting the brads from the top and back of the frame you end up with a smooth brad-free frame side like it is shown below. By using the bevelled mitre cut to join the sides and back you have a seamless joint.