As I look at that again, I think it looks like there's a slope toward the back of the shelf. Might make too shallow an area to fit the fermenters.
If so, you might look at the door and see if there are shelf brackets on there that interfere with the closing of the door.
If you do need to construct something, all you really need is something to bring a piece of plywood or similar to a level area.
Here's my fridge; if you look closely you can see a piece of stock on the bottom that I shimmed up to accommodate the two fermenters. I didn't have to bring it up very high until the door would close. You may find you don't need to bring a platform up to where the compressor hump is, just enough to allow the door to close.
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