I usually just stack up circle-cut vegetables to make a cake shape, with a baby carrot “candle.” I also have stuffed a bell pepper with lettuce and other veggies and used that as a “cake.” Just don’t let them eat the whole pepper.
I am sorry for your loss. For now, spend as much time as possible playing with and snuggling your bunny. You can offer him his favorite foods to encourage him to eat more, and provide new toys. Stuffed animals can make good bunny comfort buddies as long as they don’t chew.