This is a fixed in place adaptor,  the battery sits in close to the tool and looks as close to a factory design as possible.  You will need some copper sheet 0.5mm to about 0.9mm and small screws.  It the copper is untinned copper you will also need a soldering iron to tin the copper. 

I printed it in ASA with paint on supports as below using 30% infill.  ABS or PETG would be OK.  I avoid PLA for this as if the tool is left in the sun it may soften the adaptor.

Copper connectors, I used 0.5mm sheet, 15 mm wide at the top which steps down to 10mm, about 65mm length overall.  Insert the connector from the battery end of the adaptor and use the screw on plate to keep them in place.   The slots are 1mm wide so there if a level of flexibility with your choice of copper sheet.

Tinned, connector, the dirty looking parts cleans off the solder.

Connector installed.  I allowed for screws if required to hold the connectors in place but I did not need them and the longer part of the connector was a tight  fit.

Fitting it to the tool, I removed just enough screws to split the handle slightly (small torx tool for the screw heads)

Splitting the tool to allow the “ears” on the adaptor to fit into the Ryobi battery clip. 

Adaptor installed. Note I changed terminal cover slightly after this and it only 4 screws securing it.

Tool with battery.