if you have loads of plastic bags full of soil , why not just put 3 canes in wigwam style and then 3 runner bean plants , or maybe french beans , so much nicer !! and shorter , and just use them as is ... pop a few holes in for drainage , and you can put them where you want . You can also put some spuds in a few bags if you have enough !!!
Once you have cropped whatever you grow in the bags , it is then an easy job to just empty the soil onto your existing beds .
cheers
Russ