My experience is to use air cored coils in preference to toroid types if
possible for high power applications. If you must use toroids keep them cold
by using a good high pressure fan or preferably insert the LPF into a
portable refrigerator. (available at Halfords)
In LP filter applications running the new big super amplifiers of several
k/watts use forced water cooling coils and large home made fibre
glass/copper clad board and teflon insulation for the capacitors. This
assumes you are using a square wave derived TX with very strong odd order
harmonics 3/5/7 th etc.
Sheets of copper clad teflon or fibre glass board are available in 8 feet x
4 feet size.
 de G3KEV