Hey harrison
Check out the cheap stores. Suitable frames can be bought in an array of shapes and sizes. they dont have to be wooden either to look the part. The following oval picture frame was bought for a few dollars. I fill up the middle with white polystyrene trays ... the type meat and veg come from. this would be perfect for you to cut out a crator from. Very easy to cut and is light in weight.
All is fixed in place with a little weight and white glue. leave the glass in for strength. Then cover the top with white glue mixed with spackel so no major cracks come around. As slodder suggested ... mask the base first ... this will come in handy when painting later.
Below is a base for my M18.

And the same style again only smaller dimensions

Hope this helps some!