American Technical Ceramics (ATC) makes capacitors specifically for high
power, low loss RF applications. You might see if you can get some of
their products.

My guess is that you'll find ordinary NPO ceramics to be good enough if
you use multilayer ones, rather than disks, of decent physical size. The
critical spec is the ESR if you can get it for candidate ceramics. I
don't, however, have personal experience at running them at this power
level.

Roy Lewallen, W7EL