I've been playing with the spreadsheet too and think I see, possibly, it working @300a with one $40 (in bulk) 165mm 26u sendust toroid @50khz and 3a/mm^2:
CS1625026E - Sendust - Powder Cores | MH&W Direct with 20 turns @17uh effective, 72% duty cycle.
CSC . Will take about 7lbs of 26 gauge magnet wire (litz would be nice), some sort of 3d printed wagon wheel insert/seperator would be helpful for winding and reducing capacitance. The strandiness will need to be leveraged because the window fill factor is considerable.
I'm not sure about welding cable, there are some studies on
unvarnished stranded vs litz though (more headhurt). I would lean towards stranded magnet wire at a minimum, even if the strands cant share current perfectly, they will share heat load fairly well.