"I then plan to try to measure a variety of setups ranging from
homemade aluminum foil & iron paint to soldered copper and welded or
bolted stainless steel, to identify ways for everyday people to
cheaply create shielded environments that are actually effective. I
would like to find a way people can use off-the-shelf supplies to make
environments that are isolated from DC to light, if desired."
Sounds like a fascinating project. You should take a while do define your goal. Do you want the room to be magnetically (static; DC?) shielded as well? If so, look for iron sheet or foil to back your aluminum-foil walls, floor, and ceiling. However, iron foil is very expensive, compared with household-grade aluminum foil.
Sheet metal, even if it is thicker than foil, would probably be cheaper than this.
You may also want to install a layer to absorb the radio waves emitted inside the room. There are commercial materials to do that, some form of iron compounds, known as "ferrites". Google 'ferrite powder' for some examples. There is also 'ferrite paint'.
Take a look at the episodes of Better Call Saul, for an attorney who thinks that he is somehow allergic to electricity. He lit his room with gasoline lanterns. Lighting a radio-sealed room could be done with thick fiber optics (think rods, maybe 0.1" thick, driven by white LEDs through the walls. Or, drive the white LEDS using batteries, inside the room itself. (If you are willing to accept the small static magnetic field of the DC current flowing; driving the LEDs using coaxial cables should greatly reduce even that effect.)