Thanks for the comments guys.
I chose the delrin because I had some left over from a previous project. I didn't see that being used anytime soon, so I decided to use that instead. If I like how it shoots I will have an aluminum back made to replace it, when the need arises. I have the know-how, just lack the machines to make it, but I can't take credit for how well it came out. The internal threads are amazingly clean.
If anyone wants the drawing, I could probably offer them. I want to see how it shoots before I do that though, just so I know what people should expect. From the looks of it though, it should be a good experience.
EDIT: I would like to give credit where it is due, but wanted to wait for permission before posting anything.
cs900, from PBN did the machining of the back and inserts. He has made parts for me in the past and I have only good things to say about the results.