I have a set of three wooden steps resting on concrete outside a door. The
structure is completely enclosed, but with lots of cracks where hornets
can
get in and out.

I am afraid to try to remove any of the boards to gain access. I can
stick
a tube into the interior through a crack

Wait til after dark, so all the hornets are "home"... If you can get the
spray into there , thru a crack(s) just use the can and let 'em have it with
about half the can (stay "upwind)..then head inside.
If you can't get the spray inside that way, slip the "tube" you mention into
the crack. Use a can of Walgreens "Flying Insect" killer (not wasp spray)
and shoot it thru the tube.

Do it again the next night..etc til they're gone. I did this for a nest in
a roof/soffit. It took 3-4 nights to get em all