I thought I would give some RL Hindu perspective, just for reference' sake.
Historically speaking, Rakshasas are on par with the Hindu warrior caste, called Kshatriyas, to the point that the Rakshasa wedding (which involves abducting the woman and putting a brutal beat-down on her father and brothers) is also called the Kshatra wedding. There is a strong sense in which Rakshasa almost just means 'hostile enemy'. And Kshatriyas aren't all sunshine and rainbows, either. Their dharma considers it necessary to be cruel, hard-hearted, strong, and JUST (which, if you read the epics carefully, Rakshasas also usually are, UNTIL it comes to their personal life, where the power tends to to them).
There are also different levels of Rakshasa. Ravana's court, is widely considered quite noble and is closely related to celestial guardians (the Yakshas & Rakshas). For instance, Ravana's chief wife is on a VERY short list of ideal traditional women in Hinduism. So theme is that these are intelligent, noble, devout individuals who are frequently attracted to wise people but are unwilling to listen to their advice if they are surrounded by flatterers with more palatable news. On the other end of the spectrum from Rakshasa royalty is the lowliest folk spook in the forest who frightens people and harasses ascetics by interrupting their sacrifices (throwing bones and blood into the fire, and so forth) without causing much in the way of physical harm. Rakshasas can fall just about anywhere in between.
Now, to religion.
Their religious practice is more or less directly opposite of normal Hinduism. I don't want to use the word 'tantra' because that has . . . connotations, in the West, but we could definitely call it 'left-handed' Hinduism (this is an actual term, I didn't make it up).
Most Rakshasas in the epics are shown worshipping Shiva, the god of Destruction. Today he is the god of dance (he dances the world out of existence to make way for new life), sages, and mystics. Originally, his name was Rudra, and he was (like the original Apollo) a plague god who set upon his victims with a bow and arrow. Like the Rakshasas, Rudra would have been ritually held at bay while Vedic rites were performed, and his presence at certain events (such as weddings) was seen as an ill omen.
Evil dudes (including Rakshasas) in Indian epics also tend to worship Brahma, the god of Creation -- who is RARELY worshipped by Hindus, because in many schools of thought Creation is considered the exact opposite force as Enlightenment. This would link into the materialist tendency some of y'all have mentioned.
In terms of rituals? Well, if Hindus throw uncooked white rice and ghee into their sacrificial fires, Rakshasas use offerings of bones and blood (again, worth noting that meat has a very strong association with both warriors and Rakshasas, in India). If Hindus bathe their deities in milk, the Rakshasas bathe theirs with blood.
Socially, Rakshasas break religiously motivated social norms by eating meat (Hindu warriors are actually encouraged to eat meat, but Rakshasas eat taboo animals like dogs, cows, and Humans); they steal adulteresses to wife (or women who poison their own fathers or husbands to be with said Rakshasa); and they are usually active in the aranya (uncultivated lands, such as forests) at night. If good Hindu warriors are associated with bulls horses, bad Rakshasas and Asuras are associated with wild water buffalos (Mahishasura) and wild asses (Ravan's brother, Khara).
In conclusion, Rakshasa customs are directly parallel to their neighboring Human customs, but they would subvert them along the lines of what would be considered pure or polluting. Of course, this need not look necessarily or explicitly Hindu. Be creative. But it's more or less a practice of subverting the pure into the polluted and the immaterial into the material under the same underlying framework. As gamers, we are already familiar with doing this with good/evil and life/death sorts of dichotomies; we just need to view the symbolism differently.
