I am having the same problem and was considering an entire fiberglass covering of my own doing. My main concern was the fiberglass sticking/adhering to the plastic tub enclosure. From the short google searches I have done it says using an acetone cleaner of sorts before applying the fiberglass will allow it to adhere better.
I wanted to go with fiberglass over the 17 year old plastic tub as I think it will provide better support and many more years of service. Does anyone not agree with this?
OP what did you finally decide to do and how did it turn out? I am also not sure of how much fiberglass to buy for an approximately 24" by 36" enclosure if anyone can weigh in on that.
Also, on the outside of my small shower is a "door" that allows access to under the tub enclosure. My tub is being held up by what appears to be Lincoln Logs, so was going to reinforce that as well.
Sorry if anyone thinks I am hijacking this thread, but didn't want to have another "my plastic tub is cracked and leaking thread" when we have one already.
Curtis aka Deranged Marine
Edit: I have also tried the rubberized repair tape and a spray on type of "rubber" and they worked for a few months, but due to me not reinforcing the undercarriage one of the cracks has opened back up, not to mention how ugly it looks with tape and spray on it.