Personally, I would love a key combo to activate the selector, much like the copy tool. Maybe control-m? Assuming that's not already a key combo I've forgotten.....
What I DON'T like is that the tool sticks around in your inventory when you're done, occupying space. The mod Merging Chests ( https://mods.factorio.com/mod/WideChests ) has a selector just like yours, but when you drop it into the inventory, it disappears completely.
Maybe that's a better option?
