Great stuff, I built something similar for my brother when his goldfish got to big for their tank and needed to be moved. However, unlike yours it isn't sunk into the ground and simple stands in the corner of his conservatory surrounded with terrestrial plants and a coupe of marginal aquatic species hung in the water on the inside of the bin in pots. Its looks great and the goldfish are doing great. In total it only cost €25 for the bin, piping and silicone as I had all the other parts already.
It doesn't look great just as the bin with water but it definitely serves the purpose.
Do you have any filtration in the min pond?