There are a lot of design options available for your shadow box. I've included the files I used for mine as a free download, and I'll give some tips about how to design your own shadow box from photographs or clip art using Adobe Illustrator.
You can also create your design directly in Cricut Design Space from uploaded images or from their extensive clip art library if you don't have access to fancy design software like Illustrator.
There are also dozens (if not hundreds) of fantastic shadow box designs available on Etsy. Get a digital download from an amazing designer for just a few bucks, and customize it to your heart's content. You'll also be supporting independent artists with your extra dollars, which is always worthwhile. Go wild.
Take your shadow box frame apart and carefully measure the inside diameter. This generally won't be the same as the measurement on the label. Mine is 7 5/8.
Insert your LED strip and push it into all the corners evenly. Trim any extra lights off, cutting carefully through the copper pads.
Measure again, this time measuring the distance inside your LED strip. If you're using the same LEDs as me, this measurement will probably come out about 1/4" smaller than your first measurement. I got 7 3/8 for this one.
I found 6-7 layers to be about the right number to match with the depth of the LED strip.
The first layer is a black frame stuck to the inside of the glass that's just wide enough to cover up the NeoPixel strip, so no light bleeds directly into the viewer's eyes.
My design has a focal point of a kayaker in the foreground. There is a window-frame layer in front of the kayaker to give a sense that the viewer is looking at this scene from far away.
Behind the kayaker is a lake edge with trees reflected, and behind that is a layer with more trees and a mountain in the distance.
I finished it off with a starry sky and a solid back layer, to reflect any light back into the viewer's eyes through the stars.
I started with a high-contrast image for each layer. I upped the contrast a little more in Photoshop using the Levels tool, until the lights and darks really stood out from each other.
I opened the image in Adobe Illustrator and selected it, then went to Object > Image Trace to turn it into vector artwork.
This took a few tries, playing around with the contrast and the image crop, before I was happy with the result. You'll save yourself some time if you crop your original image to be the same aspect ratio as your shadow box.
Illustrator will make a whole lot of layers. I went into the Layers palette and deleted everything extraneous so I was left with only the black mountain and trees layer.
I used the Rectangle tool to make a 17 point rectangle around the entire artboard. I made sure the stroke was on the inside of the rectangle, and put it on its own layer so I could apply it to each of my layers individually.
This part will be hidden behind the front frame so it won't be seen, but it will help tremendously with assembly and spacing, especially for layers that don't go all the way to the edges.
Any areas in white will be cut out by the Cricut and appear to recede in the design. Any areas in black will stay as solid layers.
This is important to get your head around if you want to create a successful design. Black areas that aren't connected to anything (i.e. if the kayaker was floating in the middle of the lake) are going to be more difficult to attach and line up.
I created the simpler layers, like the starry sky and the window frame, directly in Illustrator using the rectangle tools and star tool. The star tool is pretty cool - you can change the number of points on the stars for variety and interest.
I finished up by adding a thicker (35pt) stroke rectangle for the very front frame, to block out the NeoPixel strip from bleeding around the edges.
Change the visibility of your layers to view them one by one, and export each layer as a separate .png file for upload into the Cricut Design Space.
Why Not Use SVG?
SVG files are compatible with the Cricut Design Space, and are better for some uses in that they are vector art and therefore lossless. However, they're more of a hassle to use with Illustrator. Hiding a layer in Illustrator does not eliminate it from the SVG file, so you'd need to save 6 different Illustrator files to get a clean export.
The Cricut does a great job with .png, especially at this small size, so it's really easier for our purposes.