You might try keeping an eye out for vintage/no longer made Allen set screw t-handles on Ebay etc. I don't remember if they are made out of cast iron or steel, but they are nice and strong. They are often listed as 609 handles, but technically 609M series handles are what you would want. Metric sizes will be harder to find, because they didn't make nearly as many of them, but they might work well for you if you can find them locally or with a decent shipping cost from the US (since you are in Canada).
After a quick ebay search, I can only find Allen metric t-handles currently for sale in 3mm and 5mm.
1 PER ORDER. 81210 T-HANDLE.
www.ebay.com
Find many great new & used options and get the best deals for ALLEN 81220 T-HANDLE, 5MM, HEX #609M (4G6-008) at the best online prices at eBay! Free shipping for many products!
www.ebay.com
If you want to make something DIY using very little labor, buy a couple of those vintage Allen t-handles in SAE or metric hex sizes that are just a little bit too small. Temporarily remove the set screws, and drill out each hex broach to the ideal size necessary to hold a 12" long Bondhus ball end metric hex blade. You would have a functional solution in a couple minutes. A set screw by itself can take quite a bit of torque before a hex shaft will want to rotate, so for the sizes and torques you are dealing with you wouldn't need to worry about the shaft being in a round hole. For example, most wood paddle/spade bit drill extensions use round female holes and rely exclusively on the set screws to prevent the spade bit from spinning inside the extension while drilling. They work just fine as long as you always keep the set screws tight. If you notch the hex blade shanks a small amount so the set screw can get a better bite that would be even better.
It wouldn't be that hard to make your own t-handle from scratch either, even if you only have common tools available. Take a piece of round rod, and drill a perpendicular hole most of the way through it that's sized perfectly to fit the hex shank of your choice. Drill a cross hole and thread that hole for a set screw.
The best purely store bought DIY solution I can think of, would be to buy the 12" long bondhus ball end blades and slip them inside some older style 1/4" drive hex bit sockets. The style that are actually broached for each bit size (not the newer style ones that usually just accept a 1/4" hex shanked insert bit) and also have a set screw or use friction fit to hold the hex shafts in place. Use those DIY sockets along with something like a Craftsman 1/4" drive sliding t-handle (any brand that has a centering notch on the handle bar so it keeps the square drive head centered easily and works well as a t-handle).
I wouldn't recommend anything that requires welding, because I assume you want to be able to quickly change out the blades whenever the ball end tips wear out or break off.
The Bondhus ball end screwdrivers (that vwpieces mentioned) won't work, because even the extra long versions only have about 10.5" long shafts. Many online specs show the length as being around 14.5", but that includes the handle according to the Bondhus site.
Lastly, don't be surprised if any long hex shafts you receive have a bit of a curve along the length of the shaft. It is almost impossible to give a shaft that thin and long a quality heat treatment without it bowing at least a little bit in the process.
Good luck OP.
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