Paul, I sometimes use the triangular wall-mounted brackets available at garden centers and big box stores. Depending on where you go you may find a number of different sizes, types, and colors. I like the swiveling ones because I can quickly reposition plants as needed. I attach them to my windowframes with self-tapping woodscrews.
More often, though, I just screw a few cup hooks along the top and sides of the windows. These are similar to eyebolts but are open on one side. The ones about 1 cm across are robust enough to hold several 8" baskets as long as I screw them in tight. Does that sound like what you're looking for?
--Nat
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