Corliss Lamont (1902-1995), was a socialist philosopher, and a key figure in the Humanist movement, as well as the author of books such as Philosophy of Humanism, Yes to Life: Memoirs of Corliss Lamont, The Illusion of Immortality, etc.
He wrote in the Foreword, "There has long been a widely felt need for a funeral service centering around a non-supernatural, Humanist philosophy of existence. The Humanist view... rejects the idea of personal immortality and interprets death as the final end of the individual conscious personality. The philosophy or religion of Humanism sets up the happiness and progress of mankind on this earth as the supreme goal of human endeavor." (Pg. 8)
He suggests, "A funeral service is, moreover, helpful in overcoming any sense of unreality about the death of a loved one. It brings out the finality of the parting with him, the fact that past relationships with him have been severed and that a new relationship of memory alone must be established. Rituals concerned with death are a form of art and should appeal to the aesthetic sense... they ought to be dignified, brief and reminiscent of the deep social ties in experience; they ought to avoid sentimentality... But funeral services should not try to avoid stirring up the emotions... the normal expression of grief can serve as a healthy release and purge of tension." (Pg. 8-9)
He provides meditations such as the following: "transiency and death itself are entirely natural and understandable in our universe... It is Nature's law that living organisms should eventually retire from the scene and so make way for newborn generations. In this sense life affirms itself THROUGH death... we accept as inevitable the eventual extinction of human individuals and the return of their bodies, indestructible in their ultimate elements, to the Nature that brought them forth. In death as in life we belong to Nature." (Pg. 15-16)
For a burial service, he proposes words such as, "We lay his body in that gentle earth which has been the chief support of Man since first he walked beneath the sun. To all human beings, to all living forms, the soil has ever provided the sustentance that is the staff of life. To that good earth we now give back the body of our friend..." (Pg. 21) Quotations from writers/poets such as Lucretius, Carl Sandburg, Edna St. Vincent Millay, Swinburne, Tagore, Santayana, Shelley, etc. are also recommended.
This book is a very useful one, in a field with very "limited" other resources. You might also want to explore Funerals Without God.