The bore is already 3/8" - the tap will slide right through. The centre of those hubs is actually a plastic bushing so the bore diameter in the aluminium is more like 0.5". A 3/8-24 tap needs an 8.5mm hole, or preferably a slightly smaller 21/64" hole in aluminium for a stronger thread.

BTW: Many castor wheels including Colsons are made from polyethylene or polypropylene, which are 'low surface energy' plastics that are very hard to glue to - that's why your locknut hubs failed. There are special (and much more expensive!) epoxies designed to stick to PE and PP plastics but even they don't work that well. When I make Colson hubs, I make the diameter a little larger than the wheel bore and add barbs like that Banebots hub - AND then I add epoxy as well