CLICK HERE FOR AN IMPROVED VERSION
I took inspiration from "derLowy" but recreated the part from scratch. Also, I changed the guiding angle, because it seemed better to me this way.
This part does exclusively fit the larger backplate of the latest Ultrabase models. It will probably also fit, but not very well, with older models.
Please note that this item will limit the maximum build height of your i3 Mega from 205mm to about 200mm. A new and improved version that migitates this flaw can be found here
I used the original black PLA that came with the printer to create this part.