Dammit Stupid Tarriff Antennas

I need one of these antennas but I don't know exactly which size I should get. So rather than just buy 3 of them a month ago I've been waffling :-) If I could get it to make solid contact without opening the case on this otherwise totally pristine boombox I'd just use the wire I have there now.

It doesn't have to look great it just needs to receive FM from the other room. These are both favorites of mine among the dozen or more radios we have scattered around the house and barn. I hadn't even cleaned them after bringing them both home for five whole bucks.