All the suggestions are great and as many have said it is your present wife's feelings that are most important now. Reassure her that your love for her is as strong as ever. Listen to her feelings, they are valid no matter what she thought the arrangements would be when you first married. Since she would prefer to be cremated, would she have any opposition to having her ashes interred next to you and your first wife, with you being in the center? There are also beautiful granite benches that are made specifically to hold cremains and can be inscribed.It could be placed at the head or foot of your existing plot with your first wife, or nearby. Whatever you decide I pray that all is forgiven and your family once again loving and peaceful.