I have spent the last 8 years trying to get splashes right for duck. Had the advantage of being on a flood plain next to a river so was able to build a bund and sluice at one end of the field to manage levels. I did get permission from the EA as officially you are trapping/holding water and even if it does fall out of the sky they seem to own it.
For teal you want to keep it 6" to 9" deep. Mallard prefer something a bit deeper, more like 18" plus, but they do flight into my teal splashes. If you are on a slope just pile the spoil on the downhill side. If it's not clay then buy some in and then put some cattle in there to tread it down for a few weeks.
Do you have any way of channeling water into it? Land drains? Is there a concrete yard or buildings you can channel the rainwater from? I think you are likely to need a source if it is going to be successful.
I have also played with feed for the past few years. No substitute I have found for Barley and the more you feed it the more duck come in strangely enough. I now feed 3 splashes, each 20 meters by 10 meters, about 100kg each per week from end September (we don't start shooting them until end October). Shoot every 2 weeks at high tide in the estuary and we get bags of 10 to 20 birds, sometimes a few more.
If Grouse shooting is the sport of Kings then Teal shooting is, IMHO, the sport of Gods. There is little more challenging than a good night on the Teal.