so many things...
First if your doing a transformer style balun there are single core designs to follow though they are often
not as good in bandwidth as a two core balun. See Sevik et al.
A t200-2 x2 using #14 enameled will work if wound and connected as a Guanella style and offers good
A good single core design can be found here www.hamuniverse.com/kb1lqc41balun.html
but you have to use the recommended core. Note the teflon tubing around the wire is not to protect them.
its there to create a parallel pair transmission line so the teflon is actually spacer.
Baluns are as much art as science and building them right is important.