Fairly simple, but a nice 3D effect achieved by using the warp filter on a spiral wiper.
; Layer:
; Shape: Wiper
; Name: wiper
; Type: Spiral
; Wiper size: 4
; Visible: true
; X offset: 0
; Y offset: 0
; Rotate (degrees): 0
; Reverse path: true
; Fine tuning:
; Connect to next layer: along perimeter
; Draw portion of path (%): 100
; Backtrack at end (%): 0
; Rotate starting point (%): 0
; Layer:
; Shape: Warp
; Name: warp
; Warp type: circle
; Period: 6.4
; Subsample points: true
; Visible: true
; Initial width: 100
; X offset: 0
; Y offset: 0
; Reverse export path: false