First up, assemble the two halves of the idler wheel and set aside for the cement to fully cure. Next, take a piece of masking tape and cut it to 3.5mm x 58mm. Then, along the centre line, mark a small dot every 6.6mm, starting and finishing at each end of the tape. Wrap the masking tape around the centre of the wheel with each end of the tape joining together leaving you with 8 dots around the circumference. Take a small drill and make a pilot hole where each dot is along the join line between the two idler wheel halves and then remove the tape.
Then, using a 3mm drill bit, drill out the 8 holes. A sharp craft blade can then be used to 'square' off each hole....
A square file can be used to tidy up any rough edges before adding the missing nut/bolt detail and attaching to the side panniers....
Gary