It all depends on what type of frame you have, the earlier frames had the WD/C type battery carrier hangar but the later frames had their own hangar which had an attached regulator bracket. These were not designed for the tool box.
Item 10 here http://www.hitchcocksmotorcycles.com/partsbooks/pages/265/Frame_and_Battery_Carrier
If the above is the one you have you could modify it or get a Model G one (Keeping the original as is)
Item 5 here http://www.hitchcocksmotorcycles.com/partsbooks/pages/314/Petrol_Tank_Bracket,_Battery_Carrier,_Air_Filter_and_Rear_Stand
If you need the tool box, Hitchcocks have repro boxes, I haven't bought one but they look OK.
http://www.hitchcocksmotorcycles.com/news/43935//Toolboxes_for_Pre-war_and_Post-War_Royal_Enfields
Again, there are 3 types of toolboxes, the early with the lock, the middle aged one which is like the early but with a know rather than a lock and the last type is the real simple type with a full size door with a knob. They have brackets riveted on in different places and some may need spacers fitted. If you need a photo, let me know.