I'm not sure if you have the link to this or if any of these are helpful to you, but the CC Database has a tonne of free to download barns, arenas, paddocks etc - you can find lots directly here: http://es-ccdatabase.blogspot.co.uk/search/label/Lots
There really are some great ones in there, you might be able to find something that suits your needs. Some of the indoor arenas are beautiful! (and i know i find those hardest to build when i'm building, haha!)
You may need to just change walls/colours to suit your own style but that'd be completely up to you!
Hope you find it helpful, if not, I'm sure one of our talented builders may be able to help