I did a little more research on that fraraday cage thing. From what I gather (and this is coming from a rudimentary understanding of EM shielding), if you want to retain ventilation capability, you'll have to fuse or rivet metallic windowscreen over the gap between the vertical sidewall and the roof on the soffit side (where uptake from the soffit vents would occur) in order to create what will behave as a continuous electromagnetic shield. ANy place you have a roof vent, you'll have to similarly attach metallic screening to create a continuous piece where the vent section is. If you have a ridge vent instead of buttons, mushrooms or turbine style vents, you'd probably want to run a continuous roll down the length of the ridge, brace it up, then solder or rivet a metal channel over the seam to create that same continuous surface.
YOu'll have to find some way to "seal" the door. If you have a trolley-style sliding door, shrouding the top down by an extra 6 inches might do it, but I might still be inclined to make some kind of conductive "weatherstripping" to complete the electrical seal around all the sides of the door.
If you have a wooden gradeboard, you'll have to have some way to sink the electrical shielding into the ground by a few inches to use the ground as a "gasket" to keep electromagnetic energy from leaking in. The earth is a notoriously good shield. TO that end, excavating the floor down a foot or so might not be a terrible idea if it's a dirt floor. If it's on a slab, you'll have to find some way to skirt it.
ANd that leaves the door as the weak point.
The most important part by far will be grounding it. FIrst you've gotta make all the pieces electrically continuous. Grounding straps screwed from one panel to the next should make sure that painted surfaces beneath the overlaps between panels don't create a problem with continuuity. Don't forget the roof and all of your trim. Sink a piece of copper- plated conductor bar into the ground. If you can'd do that, wire a couple pieces of re-bar about 4' long together and hammer them in, then electrically attach this thing to the shed.
I think the test will be to take an FM radio inside and if you barely get any reception at all, you win.
Is there anyone else here who *actually* knows what they're talking about who can comment on my wild ramblings?