I found a cheap way of doing this.
I went online and found a scientific equipment supplier and ordered a few "Retort Clamps" (picture below) for a few pounds each - very cheap and will fasten almost anything to a mic stand at any angle.
I think these will do the job and potentially many more around the studio.
Why not use a lighting umbrella, straight into a light stand - made for each other.
Random picture from Google…
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 ...