MG potting mix is good stuff, I grow a couple tomatoes in it each year. I tend to use cheaper brands though unless it's on sale, or buy a bag of each and mix.

Plants can certainly grow in half-composted material - look up lasagne gardening. Your idea could work, although you might not need the tubs. Why not just dig a hole, fill it with a good topsoil/compost mix and plant in that? Is your goal to eventually level this area? If so you'll need soil, not just organic matter or potting mix. What's going to happen to the tubs of potting mix at the end of the season? If you leave the stuff there it will continue to decompose and shrink and your slope will still be there. That's why I'd suggest some topsoil and not just 100% organic matter. But if you're not trying to eventually have a level, raised bed there, it doesn't matter as much.

My goal is not to level the slope, but just make it easier to plant and walk around the plants. I thought the organic matter will settle and sink and hence thought the need for tubs. I am going to try it with potting mix.

Potting mix can dry out very quickly...so make sure you keep it watered well.

I have made "dirt pockets" and planted in that. Take a bag of dirt (can usually buy for a dollar or two), push aside a small area you could like to plant in, dump the dirt there, and then plant.

I'd be surprised if you really have that much uncomposted material still. Have you dug around it it a little? I bet you have compost under a layer of the uncomposted material.

I too would recommend more of a topsoil. You could blend in up to an equal amount of good compost, because those topsoil bags can be pretty poor.

How is the water flow in that area? You would want to make sure you don't get too much water there.

Another consideration is that you actually can use it as a raised bed area, taking into consideration about the slope. The bed border will help retain the soil and you can still grow things in there.

Potting mixes will be mostly peat moss, coir, finely shredded bark, or a mix of those. The organic matter you have put into that depression will, eventually be digested, "shrink" but might be something akin to a Lasagna bed.

Here is a link that might be useful: Lasagna Gardening 101

Thanks for the suggestions. I made holes in the organic matter and mixed the clay soil underneath with some miracle grow garden soil (since I had a bag of it lying around) and planted zucchini, yellow squash, trombone squash and pumpkins.
I did not use the bottomless tubs I was thinking of using. I laid down some pink bark mulch around the plants. I have a few more plants that need to go in the ground - Charentais melons, seminole pumpkins and lemon squash.

I've accidently grown huge strawberries in aged wood chips.
I also grow flowers or Buckwheat on compost piles. With only little soil and very warm compost they grow well. I'm always experimenting

minitrucker! Please tell me the story of your accidental strawberry success in aged wood chips. Anxiously awaiting your reply. COOL!