I have a similar problem with my Birmingham Scott which I bought last year but have yet to do any serious work on. However finding suitable toolboxes was one of the issues to be tackled. As far as I am aware (and perhaps someone can confirm or contradict this) there were two types of similar toolboxes used on the Scott. They were identical except that the lid on the earlier ones was flatter than those used on later versions. Further, the same or similar toolbox was used on other makes of motorcyle manufactured at the time. One of the makers that used a similar toolbox was Royal Enfield. Hence, I purchased a pair of used modern Indian Enfield toolboxes off Ebay which appear to be the same as the later version of the Scott toolbox. There are some additional holes cut in and provision for locks but essentially I think they are the same. Having said all this I’ve not actually fitted mine yet but you may find that this is worth checking out.