When I had a car that didn't have a trunk, I used a couple of simple dodges to hide stuff in plain sight.

I've used Rubbermaid tubs that have some obvious wear-and-tear, with big labels on the lids that say "Recycling - Tin Cans" etc.

I've also put stuff in big black trash bags and secured the top with the twist ties provided. This is especially handy when travelling in a rental car.

Nobody has broken into my vehicle to steal "recycling" or "garbage." Yet.