I looked at the Giandel and agree, you are kinda stuck. A contactor that can handle the current is not cheap and as you said, there is a risk using one will fry your inverter. If I was you, I would replace it while it is still working and get something better. You should be able to recoup at least half of the money you spent on Giandel.
I suggest you look at the Xantrex models I linked to. For some reason the 12V versions are cheaper than the 24V. If your system is 12V, you can get one of these for less than $600. These inverters are UL listed which is not a meaningless distinction.
XANTREX FREEDOM X 2000 12VDC 120VAC 2000W TRUE SINE WAVEFreedom X 2000 True Sine Wave Power Inverter 12VDC 120VAC 2000WTrue sine wave 120 Vac inverter with builtin transfer switch Designed for recreational and commercial applicationsThe Free...
www.hodgesmarine.com