Nothing at this stage will fasten well, but you can try the good old 1/4 round, which comes in three sizes.
I would select some stock that is pefectly straight, prime it, paint it and then get out the mitre saw and make your cuts.
I would try to use adhesives like contact glue, two part glue and others for the fasten. Then I would use books, clamps, bricks to keep the wood in place.
this is about all you can do. I would not recommend drilling holes in the walls for fasteners at this point.
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