A grow bed of similar construction to your pond, but elevated about 3' above should work fine. The grow bed itself will provide all the filtration and bio activity you need once it is cycled. You can add a 100 gallon stock tank as a secondary sump so that flow and level in your ray tank is constant.

Bell siphons on the grow bed returns water to the sump (stock tank). A pump in sump would direct water to the pond and grow beds. Once the water level gets high enough in the grow bed, the siphon fires and dumps water back to the sump. Water level cycles in the sump as a result. Constant flow goes to the pond and an overflow line maintains constant level in the pond.

Start off your cond with conventional filtration until the growbeds get cycled. Rinsing your cycled filter into the grow bed will get it going fast.