I'm going to assume its trimmed out already with the old trim from around the TV. If so and you are having a hard time swallowing the cost of having a custom size door made and finished to match, I might suggest a net type material fastened around the lower half of the opening and a bungee run through the top edge of the net and fastened to both sides and then put board games or other OK to see stuff behind it. The net material and bungee can be bought from most craft/fabric stores.
If you want some type of door to utilize it more like a cabinet but still can't make it match the other doors you might try purposefully making it not match and make it look like it was meant to be different by making the door out of a chalk board or dry erase board or picture frame or have a map placed on it, or maybe make the door out of tinted glass.
