Need more contrast for unusual studio project; color gels and filters?

For the last year I have been working on a project photographing shadows of middleschool students on a sheet. On the far side of a room (23' 9" long 12' wide) there are two strobes pointed at the subject so that there are two lighter shadows that when they overlap form a third darker shadow: http://www.flickr.com/photos/66957614@N00/158244746/. I want the darkest shadows to be in zone II or III, and have the unshadowed sheet to be in zone VIII or VIIII. However, this has been very hard to achieve non-digitally because there is really only a three or four stop difference between the darkest shadow area and the brightest part of the sheet in the studio. Unfortunately, the room is small, has low ceillings and is the only place where I can photograph these kids (the basement of the afterschool program I volunteer for). So far I have been able to get workable negatives and do lots of intense burning and dodging in the darkroom, but my process has to change given that Agfa Rodinal is no longer being made. I decided to consider this an opportunity to figure out how to get even better negatives than the ones I was making before
Based on tests recently redone by Iris Davis and Chris Johnson (author of The Practical Zone System) I decided to go with HP5 developed in HC 110 1 to 7. Iris showed me their test prints and I really liked the way the grain looked with this film developer combo. Big thank you to Iris for showing me the test prints! I did tests of my own to try to figure out how much more development time to give the film. However, N+1 and N+2 are not enough and at N+2 my shadows start to move ALOT more than would be expected--more than a couple of stops. So, my thought is that I need to create more contrast in the studio at the very least AND maybe go with a different film/developer combination IF I can't get all the contrast I want in the studio set-up due to the presence of a big white sheet bouncing lots of light around in a small room. I would prefer a film/developer combination that will give me large grain and lots of accutance.

Other things to consider: I have used black cloth to covor the walls and ceiling on the half of the room where the strobes and my subject are. I am photographing from the other side of the sheet, the walls and ceiling in that area of the room are not covered. The strobes are the only light source, and I instruct all of my subjects to stand as close to the sheet as possible without touching it in order to get a reasonably sharp shadow. I believe that the light is bouncing off my subject back onto the sheet where the shadow is because when I photographed a student whering a jersey with a white number eight on it, the 8 showed up as a lighter area in the darker shadow.

A computer engineer friend of mine mentioned using color gels over my strobes and a color filter on my camera to increase contrast but I haven't been able to find much information about this. Any thoughts?

 

do you know of any such materials that can be found 12 feet by 12 feet? I searched for something to use other than a sheet (in San Francisco) that specialize in making backdrop screens etc. I tried fabric stores and even places that primarily deal with the motion picture industry and couldn't find any material big enough. I also tried to get in contact with some gaffers, but to no avail.