The UEB Literary Committee agrees that you should use a transcriber-defined shape indicator followed unspaced by the letter “o” or, as suggested in UEB, use a word or initials, such as “os” meaning “oval shape” or something similar. (UEB §11.7.2)
@$os
Also, if a filled oval appears in a sample ballot, a committee member mentioned that you’d then be able to use the filled shape indicator. (GTM §14.2)
@_$os
We hope this provides some direction.
Dan Gergen