Hey All! I thought it would be good to collect the side-impact solutions from different folks. So far this is what I've seen:

Plavan: 1/8" thick 7075 aluminum sheet (T7 temper)
Mulry: steel plate (didn't see the thickness)

Other options:
Impaxx foam (https://www.rollbarpadding.com/product/id-69) - thanks Bob for the site and suggestion ("For a door I would look at 24" x 40" x 4"thick $200")

I think the best solutions will involve an anti-intrusion panel + impact foam. So...

Me: 1/8" thick 7075 aluminum sheet (T6 temper, very similar to T7, slightly lower elongation %, less expensive than T7), coupled with this (2006-2007 Impreza FRONT pad):

2006-07 Subaru Impreza Front -5.jpg

I figure if the OEM's use this to mitigate front end impact, then it should be a suitable foam for me. In hunting around for this I found a wide variety of thicknesses, so I went with the thickest one I could find (4.25") that didn't have too much bow in the backside. The idea is that mounting it will be tons easier if the backside is relatively flat. I'm going to sandwich it in a composite sleeve and bolt that to the aluminum panel at the height of the main upper door bar - something like this:

IMG_4071b.jpg

I'll trim the quarter panel to move the pad further back so that the thickest part is in line with the driver. [Also note that the aluminum shown here is not the 7075 sheet I'm using.]

As an added bonus, this puts the foam at the same height as most bumpers (i.e. same plane as the upper half of the wheel/tire). You can pick these up from your donor car if you've got a 2006 donor or you can get them on the cheap from scraped cars. Cost: $50 ea. (shipped). I got two since I'm a firm believer that the passenger should be as well protected as the driver (years of instructing has sensitized me to the shoddy consideration often given to the poor bum riding shotgun). You could also double up on one side if you wanted more vertical protection.

FWIW, these are some notes I have on a range of OEM options for others to consider:

Subaru Impreza (2006-2007) FRONT pad - very flat along backside, quite thick
Subaru Impreza (2004-2005) FRONT pad - very flat along backside, modestly thick
Subaru Impreza (2002-2003) FRONT pad - pretty flat along backside, bit curved, decently thick w/recesses

Ford Focus (2000-2004) REAR pad - fairly flat along backside but ends taper in, decently thick
Scion Xb (2008-2013) REAR pad - curved backside but flattens out easily - bit thin though
Infiniti G35 sedan (2007-2008) FRONT pad - curved backside, looks to flatten easily once ends are cut off - tick thicker than Scion pad

Hope this spurs some folks to share their side impact solutions. I think those of us with wheel-to-wheel interests would benefit from sharing the pool of knowledge.

Best,
-j