[Johan]

Put a big button on the dash with "START" lable, then wire it to the alarm or horn . Thieves will mess with the button for a while at least, and might even set off the alarm or beep the horn.
A separate switch for the fuel pump can help too.

Optima makes a gel cell battery to fit your tray, and it is the same one run in the Lola race cars I think. I have a spare if you want to see if it fits or take a part number.