My 8" fits neatly into my Lancer. The OTA sits nicely at the rear of the boot nearest the tailgate and the base sits on the back seat. I can get lots more stuff (eyepiece cases, containers with batteries, binocular cases, clothes, food, charts, computer, chairs, table, blankets....) into the boot and the back seat and have room for a front seat passenger as well. I can even keep my tool boxes and various junk in the boot.
Strangely, the OTA does not fit as well into the boot of the Ford Futura? There is a well in the boot compartment and the OTA doesn't fit neatly into it. I reckon you'll do fine with your Astra. I understand that if you drive to Bintel Melbourne, they will let you try a demo model for size - on the basis that if you drop it, it's yours and payment is due!
Cheers
Eric
