I am biased, but I am very pleased with the Pletscher mousetrap rack that has been on my UO-8 ever since I built it up for my wife more than 40 years ago. She got intimidated out of riding in traffic several years later, so I built her a mountain bike and took over the UO-8 as my commuter/beater/transporter.
I flattened an L-shaped reflector mount and put the brake mounting bolt through one hole and one of the Pletscher clamp bolts through the other, to eliminate the infamous downward slide problem.
Rigidity is no problem -- I have since added panniers and easily carry 20 to 30 lbs of groceries. As a UCLA student, I used to carry a stack of textbooks on a Pletscher rack. The main problem was bike frame flex (my first Capo, which had double-butted 531), rather than any flimsiness in the rack itself.
__________________
"Far and away the best prize that life offers is the chance to work hard at work worth doing." --Theodore Roosevelt
Capo: 1959 Modell Campagnolo, S/N 40324; 1960 Sieger (2), S/N 42624, 42597
Carlton: 1962 Franco Suisse, S/N K7911
Peugeot: 1970 UO-8, S/N 0010468
Bianchi: 1982 Campione d'Italia, S/N 1.M9914
Schwinn: 1988 Project KOM-10, S/N F804069