Selecting power resistor
One of the best forms of power resistor to use for this sort of '10s of watts to modest temperatures' mains heating application is a filament lightbulb.

Wire a bulb-holder inside the space to be heated, and arrange a fan to circulate air in there. Obviously these have to be rated for your target temperature, but you get types for intended for ovens.

Consider the advantages. They're already designed for mains, and to get hot. They're self-indicating. They're inexpensive and available. The bulb holder is already insulated to mains standards from its mounting. And there's a danger of broken glass, which encourages you to be careful.