Consider using some of the removable hooks, tape, or glue to attach blackout cloth on both sides of archway. Best option would be to have velcro on sides and top, with plenty to "bunch up" on floor. Alternatively, a piece of 1/4 x 2" molding (from Home Depot or similar) can be attached at the top of the archway with 3-4 screws, then use velcro to attach blackout cloth (I've used this method for hanging handmade quilts). Consider a length of 2x6 across the bottom for added light blocking at the floor.
There are a number of tapes and caulks that are removable (often used for winterizing homes) that may be useful.