“My sliding window side to side is around 21" wide. Can I fit this into it? I read that windows have to be at least 23" wide for this one. Why must a window have to be 23" wide for this unit at 19" wide? I do not want to buy a slider/encasement unit, as they are more money and I want to take it with me when I move [window a/c units are more popular and adaptible]. I assume all I need on the sides is insulation, right? My landlord will install this and he has a special window that fits window a/c units. If this 19" is too wide, what size do you recommend?”