Have a look on ebay is probably the best bet. I would probably buy just a straight piece of ferite and then cut and glue bits together to make the shape I wanted. You can buy a ready made one from fluxon. To be universal I think it needs a larger gap and the tips shaped. See other designs for ideas. Using a 45 V supply and the large fluxon C shaped 16mm gap and just the wingdings as factory Fluxon. It will do a 223 in 1.2 seconds. I think this may be a little fast. If I used the smaller ferite which has a 12 mm gap I think it would be even faster. The part of ferite presented to the case is a round section. Perhaps a shaped square section may be better. Getting rid of the water cooling certainly makes it more portable. I think perhaps using one of the higher power induction PCBs but using a lower voltage say 30-36 V to keep the wattage down this way there are more mosfets to share the current this I think would help keep the temperatures down.