I have a fiberglass shower on a cement slab. When I pour water into the drain, water appears under the slab. Upon inspection of the drain, I find the top of the pvc pipe 1/2" below the drain cover. The packing between the pvc pipe and the 'plastic' drain body has shrunk just enough to let water pass. The packing is a rubberized material that won't budge as a single piece nor can I cut much of it out. I'm considering cutting out what I can of the packing and filling the void with 100% silicone. Any suggestions will be appreciated.

I think you would be better off to get another "donut" for your drain. Is this a mustee brand shower base? If so, I am sure you could get a replacement piece

The top strainer is labeled "FIAT". As far as I know that's the brand name of the drain unit. Hopefully the picture is helpful. Is the black packing material the donut you mentioned? It goes quite deep into the drain body and seems to be held fast deep into the unit. The packing material is a very tough rubbery material. I'm thinking of taking a Dremel tool to the packing to remove what I can. But if something else is available to fill the void other than silicone, I'd certainly appreciate the advice.

Yes the black material is the donut. Not the technical term of course. It is a friction fit gasket. It is similar to a bi seal gasket used to join cast iron.

The hub donuts that I've found are made by Fernco manufactured from a flexible pvc material. The material of the old donut I'm digging out is a rubbery material and doesn't have any ridges on the inside or outside like the Fernco one has. Does the pvc material have enough flexibility? The Fernco one is specifically for cast iron pipe. Is there one made for my application specifically or is this generic one OK for this usage?

Contact Fiat or the pic is very similar to a mustee shower drain also. See what they recommend.

I acquired a donut from Fiat that's the same number as the one I removed. The pipe coming into the drain appears to have shifted over the years and enters the drain at about a 3-7 degree angle from straight in.
I installed the donut but still have a small leak where the one side of the donut is less firm against the drain wall. Any suggestions?