oops I couldn't even write this fast enough>>92608556
WELL I don't have a particularly good visual imagination so I tend to think of more what it would represent to me than what it would actually look like, so if you think you can interpret that into actual shapes (which might be a challenge) then here you go.
I think the occulty/bat-wings design that kabuki-akuma came up (fig a) with doesn't really at all fit with /co/s collective headcanon of Monstars personality (worth noting that kabuki never really fleshed out what monstars personality was in her original work, or really anything about her beyond her visual design). It strikes me though as the kind of wand Janna would have, you know?
Anyway, I'm thinking Monstars wand might sort of look like an inbetween of prime Stars wand and Moons wand; a mix of that bubbly sorta personality/imagination that Star has, but hidden behind walls (I guess) of formality/anxiety. But at the same time when I think of monstar I think of her having an inner strength/bravery exactly because of her anxiety (think of how Ludo is molded and made (kinda) stronger by the trauma he's been through) - though I don't know what that would translate to visually. as far as colours; I suppose red is definitely a strong colour, and purple and blue are very formal/royal. But of course finding a good colour co-ordination with Monstars outfit is kinda important too (blue definitely seems to work well in that regard).
If it's any help I chucked together some general references for you; fig B is all the actual canon designs for wands in the show itself that I can find (minus Ludo/Toffee wand I guess). fig C is the Spiral Heart Moon Rod from Sailor Moon that apparently inspired Moons wand. fig D is Magical Sapphire from Fate/Kaleid Liner (I just thought it looked cool and along similar lines to what I'm talking about).welp sorry for that verbal diarrhea, good luck if you go for it