gas cooktop

Maybe use a small rechargeable, sealed, lead acid battery - Jaycar have some listed for low $ - I presume the current requirements for those lighting and safety functions would be very low (except the oven light - that would chew it up). The battery supplier could advise on the best value charger to go with the battery.

Skeety,
I'm not so sure that they need 12v for safety cutout.
Just looked at the Cramer instructions on the Waeco site and all that is required is to hold-in the burner knob for 10 seconds after lighting. Refers to lighting with a match so that model may not have electric ignition. Check with the supplier.
If however they do require 12v and you will have 240v available, Jaycar and others could supply a small 240/12v plug pack to suit.
If you do need 12v and you will be away from 240v then an SLA battery and charger as Darian described will do. Again, Jaycar could supply both.