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Shaft coupling from 0.125 D to 0.25 square

Hi, hoping someone can help me with what I think is a pretty basic question:

I have 2 motors with a 0.125inch keyed (or D) shaft.  I need to connect it to a 0.25" square socket (vex tank tread driver).  I assume there's some sort of coupler or adaptor that could help with this?  I've been looking for most of the afternoon and haven't found any good solutions but I imagine it's largely due to me not knowing where to look...this is my first robot so I'm a bit new to the whole connnecting a shaft thing...  many thanks for any suggestions or thoughts.