Okay, let's sum it all up...
As APE mentioned, all foams are not acoustic foams. But what you need is sound absorption and deflection.
Real acoustic foams are great, but the better they are the more expensive they are. Quality 2" acoustic foams like Auralex run about $5 a square foot, $3 a square foot when bought in bulk:
http://www.bhphotovideo.com/c/produ..._SINGLE_Studiofoam_Wedgies_Charcoal_Grey.html
http://www.bhphotovideo.com/c/produ...DGIE_24_Studiofoam_Wedgies_Charcoal_Grey.html
Acoustic blankets are also a terrific option - and the better ones cost as much as acoustic foam, although it's very nice to be able to fold them up and reuse them for years as needed.
http://www.bhphotovideo.com/c/product/140104-REG/Matthews_329040_1_Sound_Blanket_with_Grommets.html
http://www.filmtools.com/filmtools-furniture-pads-black.html?gclid=CO3ejfi86LsCFSRk7AodIEAAYQ
What you need is anything that will absorb sound. Moving pads, although not as good as real sound blankets, are an acceptable substitute. Bring pillows, quilts, etc.; anything that has some "soft" density. When you hang the sound blankets, moving pads, quilts, whatever from the C-stands
DO NOT pull them tight; leave lots of folds. Also,
DO NOT put them parallel to walls, put them at angles.
Get as much carpeting as you can - the thicker the better. Put it everywhere the actors feet will not be seen.
Part of the issue with hard surfaces like your concrete and steel bunker can be standing or stationary waves. This is common in square and rectangular rooms. A sound hits wall "A", bounces off, hits wall "B", bounces off, hits wall "A", bounces off, hits wall "B", bounces off, etc. This is like the infinite reflection effect of a mirror. This standing/stationary wave creates greatly exaggerated frequencies and exaggerated ambient/reverberant echo. The hard walls alone will create the exaggerated bathroom/gymnasium effect. Using a shotgun mic will greatly overemphasize this effect. Add in standing waves and you're in for a sound disaster.
So, put in as much sound absorbing material into the bunker as you can. Place it all at odd angles to the walls to deflect sound as much as possible. Get sound blankets, etc. as high as possible to mitigate reflections from the ceiling. Put as much carpeting down as possible. Use a hypercardioid mic rather than a shotgun and
get in really, really close; use omni lavs as well if at all possible.