This thing is an example of reusing and reshaping the popular drop-shaped RFID tag.

After some trials and errors, I found the dimensions to draw a perfect profile with a handful of primitives (see the attached figure).

I tested such profile with RFID tags from Yarong Tech (link), and another spare tag (don't remember the vendor) and they perfectly fit. However, there are many similar items from different vendors, and they may vary slightly from few hundreds of micrometers. I suggest printing the attached frame to test how your tag fits and eventually apply an offset.