Hey Proboscis,
20 inches off the ground -
20 inches is great if you have skunks that might bother the hives 20 inches is too high for them. But there are other ways to discourage skunks.
20 inches may be high if you have one or two deeps and then add 3-4 supers. Getting the top supers off can be hard on the back.
Cable spool - sounds good if you can easily work from the sides and the back of the hive. If you have to lean over the spool to get to the hive it can be hard on the back.
Under a shade tree - SHB seem to like the shade better then the sun. If you don't have a problem with SHB it should be ok. Otherwise I would locate it in the sun if practical.
Do I need to build a place like that for my hive ?
Do what works for you.