The chucks on my Colchester Mascot are... Well... let's say; a bit Beefier than many here would be used to, the four jaw is about 450mm in diameter (18") and with all that extra diameter we have a lot further to wind the jaws when trying to grip work at either end of it's range, I mean with the jaws reversed it'll hold near on 550mm or 22" and it'll grip down to about 20mm or 3/4" when the jaws are fitted in the normal way.
Considering that the original chuck key handle is about 450mm or 18" long and the square drive part is pretty worn and about 200mm long as well which means that it's not something that one can twirl around in one hand like so many of us experienced guys do with smaller machines, there is of course the added complication that the top of the chuck is at my eye level too, so if the chuck key flips out of the socket there's a big lump of steel flying around at head height...Not ideal.

So I made this;
   

It's no leap forward in a new Renaissance but it works for me
First of all I turned up a piece of 50mm (2" approx) diameter piece of EN24 steel about the same length, simply replicating the square end of the old chuck key, to cut the square:
I could have mounted the whole thing in a dividing head to cut the square, too much hassle!
or held it vertically in a table mounted chuck, didn't feel like it!

What I did do was hold the bar horizontally in my vise and machine the first flat, then rotate the piece 90 degrees to machine the next face with the 1st face against the fixed vise jaw, then repeat twice for a perfect square using nothing more than my table mounted milling vise and a carbide end mill,
the result was this;
   

Next I went to my stash of Stainless steel handwheel handles, Before I shut down my last big workshop I set the CNC lathe up and knocked up twenty of these (Only got about five left);
   

I then cut a piece of 20mm x 50mm EN3A cold rolled flat bar and rounded the ends, nicely first by cutting roughly to shape on my bandsaw and then neatening it up on a coarse belt, giving the piece an all over sanded/brushed finish, with a 20mm hole drilled through one end I pressed the handle boss into that 20mm hole to end up with this:    

Turning it over I sat the square drive bit in the middle and gave it a couple of nice 40mm long 5mm fillet welds to hold it all together, for a bit of "Belt and Braces" action I also plug welded the underside of the handle boss into the hole it had been pressed into, by now it looks like this;
   

Finally a nice finish was required so I decided to Chemically blacken the rustable bits and left the Stainless steel, Well I left it "Sans Stains" and buffed it up to a high polish;
   

Now when I need to wind the jaws in or out a long way i stick this doo-hickey in the hole and swing it round by the nice smooth handle which is really nice in the hand and when I have every thing in place I can either swap to the old handle (Never do) or simply put a spanner on the plain end of my doo-hickey and use it to finally tighten the chuck as the vertical handle really seems to help with control I actually prefer the spanner method to a traditional tee handle.
To give you all a bit of scale this is what it looks like on the lathe;
   
Sorry it's a bit sideways,

Anyway that's one of my recent homemade tools
Best Regards
Rick