How tight is “not tight”?
Something I would consider in your case, if the difference in size is small, would be strips of aluminum HVAC duct/pipe sealing tape.
Easy to cut 1/2” wide strips, adhere to the well cleaned inside face of the filter housing neck to fill the gap. You can add layers of tape to build up the thickness to fill the gap till the clamp will tighten to secure the filter in place.
The tape and adhesive is very resistant to high heat (I use it on HVAC furnace exhaust duct pipe joints), so can be smoothly installed and unlikely to dislodge.
Key to making the fill strip vacuum leak resistant is to apply smoothly.
