High Power .35 Impact Slug ?

Just got a new plastic .35 mag this morning and I may not be able to mill it thin enough for the 108gn. The 93 fits unmodified so I'll have to print something to feed 108 and 125gn slugs. I'm modifying the feed mechanism to take the added stress this thing will produce.

I've wanted to make an aluminum lid for the SideShot's with a grooved race around it for additional length. That plastic lid could find it's way onto a mill to get an additional .020" or so and still be fairly strong.
 
I'm thinking take .020" out of the lid and base. I worry the plastic may crack once it gets that thin if not held firmly and evenly against the rotary table... but I also have a laser cutter/engraver.
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