Coat the inside of your register collar with flat black spray paint. This will keep the light focused in the direction of the collar's circular opening, rather than reflected in all directions.
Fit the register collar to the work light. The round opening of the collar should be away from the work light, pointing towards the stage. The rectangular end of the collar will be placed around the work light. Use your tin snips to cut each corner of the work light's rectangular end. Then, manipulate the collar by folding and bending it until it fits onto the work light's fixture. If necessary, use the tin snips to cut additional notches in the collar or cut off any attached hardware to get a tight fit.
Attach the collar to the work light. Make sure the fit is as tight as possible, then secure the collar to the light using reflective silver heat-resistant duct tape.