Extra long end mills for milling foam

[Jonathan]

Probably easier to make an extender - with 20mm dia you've plenty of room to hold a 1/4 inch shank 20mm router bit in a 19mm shaft.

That's a good way to do it, depending on how deep he needs to cut. If this cutter needs to cut the surface of a tall face perpendicular to the bed, then a cutter with long flutes will get a better finish.