Who hasn't been out of balance when workng at the mast with the halyards. Well it's over on our boat. Some may htink our solution is unattractive, and it could have been made in teak, and with a smaller piece of wood, but it works, and here is the idea for anyone to consider:
Note the board between the stays!
Here it is!
And here is how I fastened it. The white part is a piece of electrical wire, the kind with only one thread in it. When tightened around a stay, the insulation will take the shape of the wires n the stay, and make sure the support never slides up or down. The line is tightened by the loops you can't se for the fluffy part, which is a line end I should have melted with a lighter flame. The wood should have had some oil or varnish, but it was made in a hurry as a prototype. It'll probably stay on for many years to come.