Polaroid 545/545i film backs are of no use for instant film products available today. These backs were for sleeved, single sheet, film only. Not for the pack style, instant films available today from Fuji.
These useless backs are easily identified by the large toggle lever on the rear of the back.

Example of 545/545i style back:

Fuji makes two sizes of instant, pack films; 3.25 X 4.25 inch, and 4 X 5 inch size.

Compatible backs for the smaller 3.25 X 4.25, instant Fuji pack films are either the Fuji PA-145, or Polaroid 405 backs.
For the larger 4 X 5, Fuji instant pack films, the Fuji PA-45, or Polaroid 550 backs are used.

Possibly sooner, then later, there may only be three, (3) individual types of instant film packs from Fuji.

In 3.25 X 4.25 inch:
1) FP100c, 100 ASA speed color
2) FP3000b, 3200 ASA speed b&w

and in 4 X5:
3) FP100c45, 100 ASA speed color

There is still some 3200 speed b&w, in 4 X 5 (FP3000b45), and some 100 speed b&w, in the smaller size, (FP100b) on some store shelves, but there are indications that production/distribution has ceased on these two varieties.