I had seen a picture a few months ago with this done and thought it was a decent idea. I like the idea that it would be a decent hiding spot. Were you thinking of locking it? The old vending machine is a fantastic idea as well and already has a locking mechanism.
Another thing to be aware of is the gas blocks can mount with a set screw, clamped on, or pinned. If you could see what the upper you are looking to purchase uses, this may help you pick a gas block.
Also be aware that barrels have different diameters where the gas block mounts; so make sure that the block you order/have matches the barrel on the upper.
Also I haven't shot one but I have heard numerous good things about the XDS as well. I just personally like the feel of the Shield better. I would say take a day and handle as many as you can. Narrow it down to several that you like and then do some more research.
Nice choice on the rifle. Fluted will be nice for the weight savings if you will be carring it around at all. Optics seem to be harder to choose than the rifle. I have a Mueller APV on my 10/22T I have been happy with it. The recticle is a bit thick but other than that it is decent for the $$. I...
Pretty interesting to hear what some of the 22 rimefire is capable of. I guess I never looked into the ballistics at these ranges. Always just thought of shooting it under 150 yards.