Cindi
My sides are for the most part, straight up and down. Instead of shelves, we capped the end of the shelves by foaming in upright rocks. These 'shelves' were then filled with many bags of pea gravel and then we planted the plants directly into the pea gravel. We have a smaller pump - 750 gallons/hr that goes into the shelf/bog area and this goes a long way towards filtering out fish waste.
We live between 2 rivers. The herons go from one river to the other, so they do fly overhead. Only one time did we lose koi to the herons (5 years ago and after that, we put in a sensor sprayer. Since then, we have had no issues.
Yes, you can spend a fortune on koi - I did and I know you can spend far more on koi than I spent. You can also get koi on Craig's list and in various nurseries for not too much. Petsmart and Petco also have koi in their fish section for not a lot of money.
I do understand puttng in catfish and bass as well. A friend of mine lives on the top of a hill and has an acre sized pond. She has the game fish - it is gorgeous and I love sitting on her deck and watching the deer and wild turkey come to drink from her pond.