if you have to use newspaper ..... wrap the outside of that with packing tape ... it will seal any air leaks to keep the heat inside.
newspaper makes excellent insulation --- but in your case where you are using it as 4 walls --- it has air leaks that lets out the precious hot air you are trying to keep inside.
you could also use bricks --- (red bricks) they will absorb the radiant heat from the light bulbs and spread the heat a little more evenly and also release a little of the extra heat during those nighttime cool hours.
newspaper makes excellent insulation --- but in your case where you are using it as 4 walls --- it has air leaks that lets out the precious hot air you are trying to keep inside.
you could also use bricks --- (red bricks) they will absorb the radiant heat from the light bulbs and spread the heat a little more evenly and also release a little of the extra heat during those nighttime cool hours.