I use round molds that have an internal diameter of 43 - 44mm for handles of 24mm, 26mm & 28mm knots with at least 2mm clearance over knot diameter.
This size gives very little resin waste, and a large variety of shape options.
30mm knots can go in the 43mm molded blanks, but the handle shape options are quite limited by not having much diameter to work with after the knot socket is cut.
It can be done, but might not be an appealing proportional shape. A knot socket needs to be supported by a few mm of thickness if it is going to stay a one-piece brush.
I have 50mm molds too, but I would only use them to create a large handle to suit 30mm and above knots.
A lot more resin waste with these molds, and resin isn't exactly cheap, so I only use those when needed for the big brushes.
Purple heart is nice, and I have no objection to turning that into a handle. I mean, it is called
"wood turning" for a reason.
Just know that wood in a wet application (i.e. hostile hot water submersion) can have some detrimental effects on it, and could result in a very nice, but temporary brush, no matter how well finished at the time of manufacture.
Stabilising the wood first (replacing air with resin under vacuum) does help, but isn't a guarantee against water ingress and cracks. Some members here can attest to that.
Plus stabilising would likely turn your nice piece of purple heart brown.
But do PM me if you want to pursue this, I know we can work something out.
Apologies for the poor photo skills and camera. That pink and black brush is actually clear resin with large purple glitter flake suspended in it.
The top appears very dark colour, but it is red and green candy colours that only become apparent in strong or direct sunlight.
Subtle built in hidden colour surprise when in the right light.
I think I'll now go turn a wood handle brush this afternoon!