And here we go with the mods, which are always ymmv. it's not a guaranteed fix for all situations.
cables(such as Schmide linked to): Yes, they help provide a workaround for the physical slot limitations, again..mods are always ymmv...
riser cards: sure, but where exactly are you headed with this? The PCIE slots are still standard, meaning you still won't be able to force an x16 card into a smaller slot without modding.
Or by "interference", do you actually mean the standard slot design?
mod all you want, I'm not wrong or dense about it. an x16 card will not fit into a smaller slot, without modding in some way, and it's still not a guarantee that it will work. The same goes for any similar situation. x8 in an x1 or x4...x4 in an x1...x16 in an x1...
So my original post stands, because that's the way that PCIE is designed. I'm not sure what the problem is here, if you want to show me a link of an x16 card fitting into a smaller slot without mods, and still being functional (ie. slot and/or card isn't destroyed), go for it.