You can find kits to build your own POT Ferrite transformers. Each includes 2 halves of ferrite core with no air gap (true transformer), 1 plastic bobbin with 10 pins. For example
I personally tested few low power builds (about 7 different samples of this kit with variety of coil turn ratios). My tests were for mid-frequencies 50-150KHz. The voltages per turn were about 0.85..1.25V per turn. The power was between 2..8 Watt. The signal form was symmetric AC square waves with 50/50 ratio. The wire was enameled magnet copper wire 0.33mm diameter. With 5+5 turns of 4.316V center tapped primary I have got output secondary voltages for multiple DC rails 28V/25mA under load (32+32 turns), 4.88V/100mA (6+6 turns), 6.7V/50mA (8+8 turns), 17.06V/40mA (20+20 turns). Primary consumption was about 0.39..0.42A. Average Volts per turn was 0.8632V/t. Frequencies were between 50KHz and 150KHz.
The reason, I used this transformer is lowest height profile I could find with documented material. The height of core above the board surface is 13mm. After assembly the core can be taped with ordinary sticky tape.
The transformer is definitely higher power than just 8Watt, I tried. If you are experienced, this kit can go to few dozens of Watts.
Other notes: When designing the coils, choose the pins of coils to be on the same side, because if some of the taps of the same coil go through opposite window, the turns count will be not exactly integer number (for example 4+4 turns may become 4+4.33 if you choose pins 1,5 and 6, BTW it can or can not be OK depending on how accurate your design goal ratios). This is valid for any transformer with opposite windows (opening in the core to let the wires tapped).
Other notes: With some effort, the row of pins can fit into prototyping breadboard, the pins are tightly pressed into plastic bobbin, but can be rotated a little bit when needed.
Other notes: The bobbin capacity is about 90 turns total of 0.33mm enameled wire. With extra effort may be 120 turns. I only used "tap in the middle" in my example above where say 32+32 turns for 28V rail have first 20+20 turns reused by 17V rail. So design your coils accordingly, so the wire will fit on the bobbin and try reusing portions of coil if you have multiple secondary coils which can have common portions of coils.
On the schematics below there is example of different but similar transformer with 12 pins together with 555 based generator and parts of diode bridges on secondary side.