As I said quite early in this thread, you could use Alt+8709 (which is the HTML numeric character reference for an empty set, a null set, a diameter).
You’ll find it in a subset of Arial Unicode MS (Mathematical Operators).
Arial Unicode MS looks very much like Arial. Here is from the Wikipedia article at Arial Unicode MS:
In digital typography, the TrueType font Arial Unicode MS is an extended version of the fontArial. Compared to Arial, it includes higher line height, omits kerning pairs and adds enough glyphs to cover a large subset of Unicode 2.1—thus supporting most Microsoftcode pages, but also requiring much more storage space (22 megabytes).[1] It also adds Ideographic layout tables, but unlike Arial, it mandates no smoothing in the 14–18 point range, and contains Roman (upright) glyphs only; there is no oblique (italic) version. Arial Unicode MS is normally distributed with Microsoft Office,