What to do a open doorway in the back of my room??

[MrDeeds]

So I love my theater room...all except for the concrete floor...and the giant hole in the back of the room. There's nothing I can really about the floor besides treat it up with big rug(Which I have done already) But, I can do something about the hole in the back. It gives me a very fluttery echo and makes it super hard on my two 10s to pressurize the room. So here's my question. What do I do about a open door way that is roughly over 7 feet tall and a little over 4 feet wide? Putting a door there isn't a option, sadly and the suggestions need to be on low budget, seeing as I just recently had a accident and have hardly any money now Any suggestions?

Thanks guys!