The transformer here would be a 4:1 Guanella balun
https://vk6ysf.com/balun_guanella_current_1-4.htmTo go from 50 ohms to 12.5 ohms, connect the 50 ohm impedance to the two toroids in series connection, and the 12.5 ohm into the two toroids in parallel connection. Depending on the power, you can probably use two FT240-43 toroids or equivalent Fair-rite.
It is best if the windings around the toroids are 25 ohm, but realistically you would need to use microstrip-like line to get that low of an impedance (the above link is for a 50 to 200 ohm transformation and so uses parallel wire transmission line which has about 100 ohms impedance). You can use two 50 ohm coax in parallel. However, if the length of the transmission line around the transformer is less than 1/10 wavelength at the shortest wavelength of use, the impedance of the transmission line is not that big of a deal, and I would just use single 50 ohm coax, something like RG-316 or RG-400 teflon coax for high dielectric strength and small diameter. Not that many turns should be required for 14-54 MHz, for example, I think 6 turns should easily suffice, which is about 0.4 m length, less than a 1/10 of a wavelength at 6 m. Try to match up the lengths of the coax on the two toroids as best you can otherwise.