Can carbon film resistors be used as heating element?
A resistor makes a perfectly good heating element for air. All electrical heaters are 100% efficient, whatever they're made of. If you want to heat a solid thing though, it can be a bit difficult to physically couple a tubular resistor nicely to the surface.

If you put half a watt of electrical power into a carbon film resistor rated at 0.5W, then it will get hot, and will stay below a temperature that's safe for it, and will deliver 0.5W into any enclosure that it's in.

Whether 0.5W is enough to raise your enclosure by 10C is another matter. As it's 10cm on a side, that's quite a lot of surface area, and would need to be fairly well insulated to get that temperature rise with that little power.


  one or two 0.5W 50Ohm carbon film ...


one 50ohm resistor is just fine for 0.5W from 5v. Two would need to have a different value, two 50ohm resistors in either series or parallel would not give you 0.5W with your limited current 5v supply.


  0.5W resistor?


I would tend to use a resistor with a larger power rating, or use several 0.5W ones, so they will run cooler. Although the surface temperature rise will be safe for it, it may not be comfortable for you. Most engineers tend to 'derate' components to put in a margin of safety anyway.

Carbon film? I tend to avoid them and go for metal film or metal oxide instead, but I think that's just irrational prejudice. As they are sold, by reputable companies, and to a specification, they should be OK. They had a well-deserved bad rap in the old days for noise, but the manufacturing has improved today.