I think it is made of ABS (acrylonitrile butadiene styrene). It's definitely not fibreglass.
I have a turbo one for sale if you are interested.

Thanks Glenn, that sounds like a great idea & it would look great, but I think I'll go with the option of either covering the hole completely or getting someone to make me a cover for it.
Still hoping to have it fixed under warranty tho.

If you can get the cover made for it, covering the hole completely will reduce airflow through the engine bay. When I mentioned what I was going to do to the mechanic they said they'd do it when they put in the new alternator. They used a spare piece of plastic they had laying around and charged me $10 extra so its definitely cheap and easy!

Given the age of your battery, I'd suggest you do go ahead and replace that.  Sadly modern day batteries don't have the longevity that they used to.  In answer to your question about how long to change an alternator, NA's should take no more than an hour of workshop time.

Screw the battery, go capacitors! Save weight with style and innovation

"The turbo engine lid is much lighter" 
 
It is not lighter, although it would keep the water out better.
I was looking for weight savings, and I actually weighed them both but found the turbo lid is 1 kg heavier than the NA lid!!
Although the top is made of ABS or whatever, pretty sure the one I checked had a steel frame and rain shield.
 

The turbo lid doesn't have a steel frame. It is completely plastic though does have metal rain shields

She is now at an Auto Electrician awaiting instruction from the Mechanic's supplier of the Alternator.
It will be either replacement alternator & a reimbursement for labour.  Or a refund & reimbursement for labour.
I'll also get him to check a couple of other little things while she is there.  And put a cover of some sort over the new alternator when he puts it in.  Just to be sure................
 

dennis the menace
Given the age of your battery, I'd suggest you do go ahead and replace that.  Sadly modern day batteries don't have the longevity that they used to.  In answer to your question about how long to change an alternator, NA's should take no more than an hour of workshop time.

Thanks Dennis, that helps to determine how much they should pay for the labour.
 

Tree
Screw the battery, go capacitors! Save weight with style and innovation

lol, Tree I'm sure you know I don't even know what that means.

Yaaayyyy.
 
Just got confirmation that the alternator definitely being replaced under warranty by an Auto Electrician instead of the mechanic.  They will also rig up a cover when they put the new one in, checking battery as well.  And fixing electrics in driver side door that have been playing games for a while.   I will have to pay something for the door part of it, but once I had her towed to the Auto Electrician I didn't have to talk to the Mechanic any more.  Gibbo's Auto Electrics took it from there.
They will have the new alternator tomorrow & the plan is to have her ready to go by tomorrow afternoon.

That's great news