with high clay you should be Ok, depending on the type of clay including is it actually clay
For smaller scale, put a 4' 1" rebar into the middle of an 8" concrete block, then fill the holes in the block with cement. Wrap the middle 2' of the bar with duct-tape &/or a tightly-wound towel - heavy, with handle.
Now hammer the everloving-b'jizis out of the bottom until it's erring to solid, corner to corner, lip to lip. All needs to be hard-smooth, sides are a nightmare. Now add a layer of the swelling vermiculite-type(??) clay used to seal commercial/agricultural ponds, and hammer that in also. Now add water & add more of the sealing clay according to the instructions on the bag. Now cross-fingers & watch...
and send pictures.
Eat a hearty breakfast, drink plenty of water & wear a wide-brim hat - I don't want your heatstroke on my concience