Dig a hole put a liner in it construct a greenhouse around it
Are you the type to build your greenhouse from scratch or to buy a pre-fabricated one that you would just assemble in place?
Ive never even looked into building a greenhouse so I have no advice there but if you look at the pre-fab ones Im sure it could be figured out. The only difference I see in adding a pond inside a greenhouse is to ensure the pond doesnt interfere with the foundation of the greenhouse. If your area doesnt have a deep winter frost concern I wouldnt expect the foundation to have much to it If it were me I would still angle my corners to allow stronger soil stability where the greenhouse needs it most (oh, by the way, one of the things I do at work is inspect foundations on new construction )
Ive often dreamed about such a set up If my pond was rectangle I would want a bridge/dock dividing it in half which would also serve as a great viewing platform. You could also make an area under the bridge/dock to house your sump, conveniently hidden