This is asking for the impossible! It does not sound natural because maybe it isn't and in judging so whats your reference anyway. If you are in dire need of some natural sounding room you can go and record it yourself. But even then, the chances that it sounds like you imagined it, are very slim.
There are very decent sounding reverberation plugins, based on artificial reverb generators or convolution-style, the last one is said to sound more 'natural' then the first one. Be it as it maybe, it really is like cooking food. You got all the ingredients for this delicious taste at hand and believe it or not to reach a satisfying result this is not enough, because if you do not mix the ingredients in the right proportion you will end up with a different dish you thought you'll get. This is exactly the same when one would mix audio, I dare say.
So treat yourself to some good ingredients and the rest is up to you, namely: taste and experience!
And looking at some of my past music productions I always find the same plugins for the 'ambient' task:
Altiverb by Audio Ease when I have acoustic ensembles (classical music mostly), the Sony Oxford Reverb in studio productions, Bricasti when I have to deal with drums and MMultiBandReverb from MeldaProduction for all-round stuff. Thats the base I start from, after that I can end up with dozen Reverb plugins which basically all are doing the same thing but just in a tiny different way.