An almost-critical sphere of fissionable material would do. Add some neutron reflecting material that can be rotated to increase or decrease the neutron reflection. Or you could have two halves of a critical sphere that are moved closer together or further apart to control the reactivity.
A moderator is not necessary if the reactor operates on fast neutrons, thermal (slow) neutron reactors are just easier/cheaper to operate and more proliferation resistant.
If you only operate it at low power levels you would not need a lot of cooling (though the reactor would probably not be very useful).
The largest amount of mass would be the shielding. So as long as you don't care about that, you're fine (and in your question you don't mention it). There has been research into nuclear powered cars, aircraft, and trains, but the size and weight of the shielding was the main technical problem for such applications.
The output of nuclear reactions is heat, if you want to convert it to electricity or another more useful form of energy, you will need some kind of heat engine, which (depending on the temperature) needs to dump 75% of the heat input as low temperature waste heat. So you will need some kind of heat sink for that. But again that is not part of the question.
edit: To be more specific, the output of a nuclear reactor is some heat in the reaction products, and most of the energy in the neutron radiation. Reactors usually also capture those neutrons in order to convert their energy to heat, so that is anoher reason why you might want to use neutron reflectors. The most difficult to shield radiation are the gamma rays, which typically require several meters of concrete or a thick slab of lead to block.