Why try to emulate the reverb of a space if you can have the real thing? Tank-FX is a website where you can upload a sample and have it played back in a huge 11m high former water tank. The sound gets sent back to you immediately, including the natural reverb from this tank:
On their website tank-fx.com (German only for now) you can upload a sample from an Akai S-2000 style interface. You can set the desired output file format (flac, ogg, mp3), and the dry/wet ratio. Your sound will be played through a set of JBL speakers and recorded by a matched pair of Røde NT-5 microphones, providing a good quality recording. I was expecting to hear a huge reverb tail and I found the result a bit disappointing the first time, but it actually sounds very good. Try it yourself or get an idea by listening to the examples on their website.
Photo by tank-fx.com