If you have a sky world and want a player to be able to build where they want they can do a command like /place and it will place a configurable block under there feet. This will work if you are flying or walking (configurable) to work for one or both. They will be a cool down time in seconds (configurable). and also a permission node to allow player to use it with that permission eg placeblock.place.
With essentials theres someone similuar, you can just do /up 1 and places a block and moves you up by one block
To place a block lower usually skyblock users place water to go below the blocks but if you really want the plugin I can make it. EDIT: nvm didn't see that it was filled ....