I built a SNES controller to VB adapter recently because someone wanted to play Hyper Fighting on the Virtual Boy using an SNES controller. The SNES controller is shown on page 5 of the HF manual. There’s a switch for turning the VB ON/OFF using a 9V 1.5A AC->DC adapter via a 2.1mm jack. Otherwise just the SNES adapter and the VB connector. Pretty simple and it came out quite clean.
Here’s the adapter with a themed controller and a VB.