Rigid bins sound great.
I save and reuse the sturdier plastic bags (like 2 cubic feet) that pine bark or other amendments come in. I use them to store soil mixes that I've made, or screened bark mulch. If you bounce them up and down several times as you fill them, they tend to stand up a little more reliably.
Turning the lip inside out and folding it down several inches makes the bags tend to stay open, making scooping easier. The soil mix does tend to dry out excessivdely unless you fold the top together or lay something plastic over it, but it breathes better if you leave it open.
(Don't store soil mix damp, especially damp and sealed, or it tends to ferment anaerobically and produce acids and other things bad for root hairs.)
(Sometimes I use plastic bags as root barriers in raised beds, or to line the inner walls of raised beds so they don't dry out as fast - especially the corners.)
When I get a bottle of laundry detergent empty, I rinse it well and cut the bottom off. It makes a good stiff scoop for soil mix, amendments, coffee grounds, lime or fertilizer. I use the sides to press down and firm soil into cells or pots. The cut edge makes a good scraper to level the soil off on a tray of cells or pots.
I keep a very big, dulled stainless butcher knife to break up and stir bags of mix, coir or peat. It's bigger than a trowel and sturdier. Used from Goodwill: 69 cents. That used knife bin is also a good source of sturdy, rustproof, custom weeding and harvesting tools.
