I would wire in a pigail fuseholder from the largest wire to the fusepanel that shuts off and on with the keyswitch. The other end of the pigtail fuse holder to the higher end of that old fuseholder.

Throw a spare glass fuse or spade fuse in the glove compartment that fits it as well.

You may have bypassed another item in the blower motor chain with your picking up your voltage source from another supply source though. At least keep this in mind. Could be the keyswitch or relay controlled by it although I do not have a wiring diagram to tell.