General Style

Render in Unity or Cinema 4D
Mess Painter in Filter Forge
Open both in Photoshop
roughly select border inside and then on the rendered layer ⌘⎇R - smooth 10, feather 10 - inverse - cut
set rendered layer on top and to 50-60% opacity
flatten image

Building Images

load both images, set rendered on top, 60% opacity
load _mask.psd
use it as a mask (⌘-click) to cut out outside form both layers (make lower one non-background by double-clicking)
add a layer style of stroke, 75% opacity to mask, set fill to 0
merge layers, resize to 400x400 before saving as .png