Menu hooks

I have an extensive menu - and to help manage it, I've split it into several parts.

I created additional menu locations and used the 'generate_inside_navigation' or 'generate_after_primary_menu' hook to add menu items below the primary menu. Works perfectly - with primary menu at the top and subsequent menus below.

However, I want this also want to do this with the slideout menu. There isn't a similar hook to 'generate_after_off_canvas_menu' and the 'generate_inside_slideout_navigation' hook doesn't work.

Any ideas?