PlexiDor Pet Doors are very well-made from high impact acrylic and aluminium. Lockable and the option of a steel security plate. From smaller cat flaps up to XL sized dog doors in Silver, White and Bronze.
Very informative website and their comprehensive brochure explains heat loss, energy saving and security. More expensive than the more widely known plastic cat flaps but excellent value for money for the security and energy conscious.
PlexiDors are from the USA.