This thread discusses Hedstrom unicycles (of which this is one). It talks about seatpost attachment but doesn’t otherwise contain a lot of good news. However, you’ve already taken a significant step towards becoming a unicyclist so rejoice!
I would take ezas’ advice about cutting a small slot down the seattube and using a regular seat clamp. A bike shop could certainly do this for you.
You could also shim a loose fit with strips from aluminum cans.