Support for AC in garage window
 

Air conditioner support I bought requires 3 long screws in "solid
material" for support. I drilled the first hole through the stucco
and found mild resistance then a big air gap then (I think) metal,
another small air gap the metal again and a little bit of wood dust.
Obviously a no-go situation. Thinking of using pressure treated 2bys
for ground contact under the window and SYP for top. Haven't googled
for screw type, sorry, but need to know what type of screws to by and
second opinion on concept. Located in Georgia, outside of Augusta,
with garage temps in the 90s where sweat splatters the wood I'm
working.
Presuming a switch from PT to SYP wouldn't be a problem. Will try to
utilize joinery at corners and where boards meet to minimize
dependence on glue, TBII, and screws. Opinions welcomed!