As one who has searched for contraband, I have a number of ideas, but without an idea of layout/structure its hard to say what will fit or work.
Obviously underfloor is out because of storage.
Are there rafters/crossbeams you could box in?
What kind of heat? Small wood burners mounted over a metal plate can keep people from looking under the plate.
PVC pipe underneath can carry many items, just offset the threaded caps to open from opposite sides. Double cap one or two and cut an opening on top that can only be accessed from inside. A little white electrical tape will cover the saw cut nicely.
One friend took a crate of Nagants and hid them in plain sight.
He took an old glass table top and siliconed it onto the top of the open Nagant crate. The rifles still racked, painted with flowers and peace signs using day glo water based paint. The crate lined with excelsior, it covered ammo belts and boxes of ammo.
A second crate under the first, concealed much of his gear,
And right on the side of the upper (rifle) box, stickers that said, “In case of emergency, break glass”.
A Boulder hippy, he entertained a number of anti’s, eating dinner inches above “real guns” they just thought was nouveaux art.