I was in this exact same situation a few weeks ago and whilst I started out adamant I was going to buy a fibreglass box I ended up with a Poly one.
I wanted a long, low and narrow box for Spanish Mackeral and whilst the fibreglass option would probably have kept the ice longer, it was 3 times the price.
I have used mine now and have kept ice for two days and still had plenty left. So I don't think it is a major issue for the couple of days of fishing we do. Maybe if we were gone for a week it might be different.
I ended up with a 200L IK200 Ice Kool box for $350.
http://www.icekool.com.au/ik200.htm
The version I got has two handles on each end and cam type clips on the front, which is a little different from the pic on the above website. I am guessing this is an upgrade to the model from when the pic on the website was taken.