Add a locked design view to the current sheet
Place a design view on the current sheet with Views > Add to current sheet, then adjust its scale from the viewport's left panel.
Views > Add to current sheet locks the design view it places ('Current View has been locked', 'View added to presentation').
The viewport's left panel offers Scale > Presentation / Design (e.g. 1 : 46.8 -> 30) and Reset Image Scale to Design Default.