This design allows you to use a 3/8" socket on a ¼" drive without adding any additional length.  I recommend printing with 100% infill.

There are two versions. One has outer dimensions of exactly 0.375" (3/8").  The _oversize option is 0.385" and is designed to be a tighter fit in sockets with looser tolerances.

This obviously isn't nearly as strong as a proper metal adapter, but may work in a pinch.  My main use of it is with my ¼" torque wrench.  I printed this in PLA with 100% infill and successfully tested to the maximum torque of this wrench (80 in-lbs).