After looking at the fuselage structure under the baggage compartment, and thinking about all the various ways I could install the smoke oil reservoir, I've decided to mount it dead-center in the baggage compartment. The support is best there, and the implications are least. There will still be plenty of room on either side of the reservoir (and on the back shelf) for tool bags, soft-sided luggage, lawn chairs, and whatnot.
I fabricated a doubler plate from 0.040 and drilled it to match the hole pattern already in the floorboard, then drilled it for four 1/4-20 nutplates -- the four points where the reservoir angles will attach to the floor. I also marked the doubler for the locations of the rivets that will attach it to the floor. (The doubler will be a permanent part of the floorboard, but the angles will be removable.) For reference, the size of the doubler is 11-1/8" x 15-1/8".