Have you seen these circle wall appliques? Basically, they’re circles with sticky backs that you apply to walls for a fabulous custom collage effect. The pre-primed wooden circles are sold in packages of 8 of varying sizes for 42 bucks.
For a very inexpensive DIY alternative, how about buying wooden embroidery hoops; prime and paint the inner hoops and then stick them on the wall with a bit of sticky back foam tape? I’d probably fill any small gaps in the wood with wood filler and then lightly sand them for a smooth finish. For a cost comparison, JoAnn Fabrics has wooden embroidery hoops ranging in a variety of sizes, starting at 90 cents for a 3" hoop up to 14" for $2.44.