I have a torroid an ebay LCR and magnet wire. Balun for HF?

[Beamin]

So I have lost my ebay 10$ HF balun that was just a tiny torroid and some magnet wire.

Is it possible to make a balun/unun for RX with my SDR for below 30MHz? Its a 82' long wire that attaches to coax and ground at the bal/un

The torroid is
3cm dia
1 cm dia hole in the middle
0.5 cm thick (side) X 1.cm thick (front to back) gray magnet like material
I have magnet wire and a ebay LCR meter like seen on eevblog.

How important is the torroid size for making this work? The whole balun unun always screws me up when going from coax to a longwire that's open on one end. That's an unun right? Coax is 75 ohm rg6.