I have a wide window a touch less than 6' across and I want to install a blind with 2" deep horizontals. The blind that was within my price range comes with five support brackets, which can either attach to a vertical, or a horizontal surface. The kit comes with two screws per bracket, (although the end "cap" brackets appear to be more load bearing, with the center 3 for stability).
I don't know the exact weight, but likely between 30 and 50 lb. [Edit: actually 20 lb.] That's before I've stripped out the unneeded horizontals from the bottom of the blind.
I cannot drill into the vertical surface, because that's the plastic window frame, and doesn't really have enough "meat" on it for supporting something this heavy.
The horizontal surface above is a thin (1/4") cake of old plasterwork, over a steel joist; possibly an I-beam.
The walls are adobe brick.
I've considered building a minimal plywood support frame, with a piece of steel angle to match the window frame. That's not outside my skill, but it's still a lot of work.
I found some magnets about 2" long, pack of 10, which say the breakaway force is 60 lb each. They have two screwholes each. I'm looking at using two magnets per bracket, with a piece of 3/8" plywood in between the magnet and bracket for their respective screws to attach. I'd check how strong they are before hanging, and possibly clean away the old plasterwork to achieve a direct contact between the magnet and the steel beam.
Does this sound reasonable? Other suggestions?