For 0.5 MHz band, Steve has a promising idea: 10mm 455 kHz I.F. transformers.
These cup-cores are self-shielding, and a big bonus is tune-ability. Most resonate
with a 180 pf capacitor, which is sometimes included, hidden in the base. The top
cup has screw threads that adjusts a small air gap, allowing about 600uH -700uH adj.
Unloaded Q is in the 100 ballpark.
I have found that most I.F. transformers can be disassembled (with care). All that's needed
is for a tri-filar 50 ohm winding to be added to the inner core. The original winding
(made of very fine #40 wire) would be left intact. The inner-core is a dumbell-shaped
ferrite, and there is usually enough winding space that the extra turns can be
added over the top...even if the trifilar winding is done with heavier wire than #40.
Yes, it is a tricky job - these cans are small and delicate, but with dexterity it is do-able.