If you want to put up a workbench in the swamp, set it on the ground first, then put a floor piece up a tree, break the old bench and then put it in the tree. Technically archers will still be able to target it, but it's rare that they do.

Alternatively, use the hoe to raise a bit of ground and put a campfire on it.

Remember to remove some of these when you get to the point of hunting wraiths for chain.