It's much easier to block the bottom entrance and put two shims under the cover for a top entrance...http://www.bushfarms.com/beestopentrance.htm
My stands are all 3 1/2" tall... except for slopes, where they run from 3 1/2" to whatever...
My hives being 18" up dont get grass, mice, ants, earwigs, or other creepy crawlies in them. Bees prefer 15ft up (5M) I think its best to be only one decimal point out.
I have Top vents open when temperatures rise high enough to require it to reduce fanning, but no top entrance, definitely not in winter.
The height of these stands was determined by a good working level for inspecting the brood box. I dont have back trouble but I'm willing to spend inorder not to go there. The anti-pest benefits were a bonus... but maybe worth it for others