Amidon T68 cores work great to 100 watts.
low HF use T68-2 (red) above 20 meters use T68-6 (yellow)
I assume the 100 watt limitation is due to core saturation?
What could you use for 300 watts + ?
Larger diameter cores...... or maybe stack 2 or 3 ?
Or possiblly a stack of larger beads? ( like the 1" diameter 1.125" tall )
or do they know the Q down maybe.........
Different type altogether......?
Don't try?