My helios finder is too big for my FED2 and it slides off my Kiev. If the gap is quite big, you could try cutting a square from a beer can (soda can if you must) and use this as a shim under the helios. You could fold over the leading edge of the aluminium to catch on the front of the finder so that it drags it into the hole, and fold it down at the front of the shoe. If it's still slack, you could then crinkle the aluminium to act as like a spring.
I was quite pleased to find that my Helios finder could stay on my Kiev 4a and i could still put the case top on! Very useful.
Dave