Why not use a lighting umbrella, straight into a light stand - made for each other.

Random picture from Google…

[![enter image description here][1]][1]

The lighting clamps generally come with adaptors &/or inserts so they'll fit a mic stand thread [either size], or otherwise they'll just drop right over the end of a standard  mic stand & clamp to the straight rod underneath.

**…or… gaffa tape.**

  [1]: https://i.sstatic.net/1CpZQ.jpg