The dimensions are in the RAC Blue book as I recall, and I do not have my strut to hand, in my office, to measure for you.
Any material will suffice altho' it is meant to be black in colour.
I have seen a wide variety of materials used but personally made mine from a the plastic used for 'house-for-sale' signs, bolted to some aluminium angle which fastens to the grill. The plastic strut was then covered in black tank tape. Sounds a bit 'Heath Robinson' but still looks presentable after six years and does the job.
I'm sure there are dimensions on this site somewhere, either under Competition/Sprinting advice, or failing that on Alex's site, somewhere.
Andrew