Hi UK,
Saw one several times many years ago used to haul lumber on a Model A Tudor.
Best I can remember was:
It appeared to have (4) approximately 3-1/2" diameter black rubber suction cups resting on Model A side roof top metal which supported two (2) transverse, (side to side) about 3" wide by 1-1/2" deep stained and varnished oak strips.
On top of the transverse wood strips were two (2) longitudinal approximately 2" wide by 2" deep stained and varnished oak strips, (front to back), over the driver's side and the passenger's side, secured with four (4) through bolts, (one on each corner).
These longitudinal wood strips had (4) approximately 2" wide tie-down straps, ((1) each corner), which passed through the tops of the glass windows of the four (4) doors where each strap had a friction type buckle to tighten and securely fasten each strap.
Never saw anything else but lumber, but I guess a sheet of plywood could be attached on top to carry luggage, groceries, packages, and other miscellaneous items ......... but never a dog kennel if one intends to run for President .... like Mitt Romney.
Hope this helps.
Added Later: FWIW: The same four (4) above mentioned corner bolts were attached to the four (4) suction cups ..... this rack was most probably homemade ...... saw several other similar manufactured racks on other vintage cars but with wide steel hooks covered with rubber to grab the underside of the rain guards, but never saw another one exactly like this one on this Model A.