A few hours ago I was standing in my garden looking at the hole were I will put my first pond, scratching my head thinking of my subsoil test where 70 mm is large stones but mostly silt and 3 mm is clay. With no pigs in sight until septemberish at the earliest, and plastic is OUT OF THE QUESTION GOD DAMNIT, it hit me as a tonna brix: NEWSPAPER! I thought "Yo! Half an inch of newspaper will suffice for sure!" Later I've been thinking it might be a good idea with a layer of cardboard underneath it to protect it from the stones, then I was thinking better to gley it underneath by using some organic matter beneath that as well. And then, on top of it all, fill it up with a few inches of that silt.
Use the glossy kind that a lot of the ads are printed on. To get that gloss finish, they add a goodly percentage of clay to the paper mix. It's not going to be impermeable to the downward passage of water, but it will sure slow it down -- maybe enough to work for you.