I'd go for a rack also. This time of year garage sales are popping up, and just about every other one the wife drags me to has at least one bike rack. Or flea markets are a good place to find them also. I bought a three bike rack for $10 at a flea market.
As for security I use a cable lock fed through the hatch hooked inside the car so it can't be removed, and padlocked around the bike. Not high security but good enough if you just need to pay for gas or grab fast food etc. I wouldn't leave a bike on over night if you stop at a hotel though.