I have gotten some large 125mu binocular cores from the dumpster here
at work. If they are a good choice for your balun then I will make a
portion of them available to people here on this board for shipping
cost. The holes are about 1/4" with a LOT of ferrite around it.
Should only need I think 1:3 of small wire for a nice unit.

If somebody who knows more than me would like to make comment on a
good design then please do.

regards,
NEO