So I ordered new shields from Angela Instruments at $1.49 each: Black aluminum. They fit tightly on the base of the tube sockets and are much sturdier than the originals. No way these things are going to fall off.
BTW, I played a gig last weekend without any tube shields and didn't notice any change in the tone or performance of the amp. Some players claim the shields have a small effect on reducing feedback, which would make sense if your tubes are slightly microphonic. Other players say the absence of shields helps with heat dissipation and tube life. I decided to go with the tube shields because they protect the tubes from random physical damage (and my preamp tubes are nice NOS Philips and Sylvania) and because they also have a spring inside that acts as a tube retainer keeping the tubes properly seated in the base.
And I know it is a bit silly, but the new tube shields look good in the amp. The old gray shields were kind of grubby and they sat askew on the tubes.