I always thought this illustration posted by Mick West in the "Cube in a Sphere UFO's Seen by Navy pilots. Radar Targets?" thread, linked to by Mendel above, had some similarities to Ryan Graves' (and perhaps @SMdelta's?) description. From a 1972 patent:

The basic idea of a radar reflector in a balloon is much older, with this patent going back to the 1940s

"Air-borne corner reflector", US patent US2463517A, https://patents.google.com/patent/US2463517A/en

I don't think we've found evidence of such items being used (very similar-looking foldaway/ inflatable radar reflectors are available for maritime use, but they're not lighter-than-air AFAIK- they wouldn't be encountered in flight). We've certainly seen examples of radar reflectors of this shape suspended from balloons,

External Quote:

The TWZ Newsletter, Emma Helfrich, 5 December 2022 https://www.twz.com/china-seen-using-radar-reflector-balloons-to-defend-key-targets

Back to the reflector-in-a-balloon designs, the basic concept is reasonably simple, and other types of lightweight reflector structures visible within balloons have been tested,
e.g. this University of Arizona, Steward Observatory Radio Astronomy Lab prototype for their Large Balloon Reflector Project

"The Large Balloon Reflector Project", 2014 http://soral.as.arizona.edu/LBR/ (note, website flagged as insecure).
The project has progressed, although later balloons appear to be of metallic appearance, not transparent.
Maybe there are (or were) other research (or military) balloons with a similar appearance to this or the earlier patent designs.