Albert, doesn't foam act as an absorber? I would think you'd need something harder and more reflective like ABS plastic or wood. Where do you find a hot wire cutter?
Foam is softer than wood, but with several coats of paint the surface of my diffusers are quite solid and very effective. Remember, RPG has now opted to offer their diffusers in foam as well.
Although mine were created 17 years ago, the idea remains the same.
Is there a plan you used to determine the configuration of your diffusers? Also you indicated they were foam, what size foam pieces did you start with?
I did not do this, I bought from a DIY guy.
My foam diffusers appear to have been copied from the original RPG Acoustics QRD® 734 model. I don't know how he did it, only that they work extremely well, they're lightweight and inexpensive.
I'll bet there are places that cut foam packing and such that have a hot wire machine.