I've read about using bags of garden or potting soil as planters/mini raised beds. Poke holes or slits in one side for drainage, lay down, then cut a small X for the transplant, and water.

The garbage bags won't hold up to the sun.

I used three of the Mylar (?) bags that Starbucks puts their grinds in. They will hold up well for a couple of years, or perhaps longer. I think they are going into their third year. I just did it as an experiment to see how long the bags would hold up and with no other goal in mind.

5 gallon bucket for under $3 at Home depot and Lowe's
10 gal $4.50 at Walmart