And I've just worked out that you click on the flat gray space that remains after a mined rock lump to build.
Thanks for the reply and for a fun idle game.
I did find that later in the game the Hauler bots would go back forth as a big group between newly release resources, which could mean they take a while to drop any off. Not a big issue as it's near the end when I'm just waiting to get as much sap as possible, but did make me wonder if more smaller haulers, or smarter haulers would be good.
Thanks again!