Custom head for U-Net

Hey, everyone!
I am struggling a lot to create a custom head with nearest neighbor interpolation and to do segmentation on CamVid according to a question in Chapter 15. I cannot put a custom model in unet_learner. It requires only particular architectures given in model_meta. So, could anyone give, at least, a clue to how this should be done?
I’d really appreciate. I have spent a lot of time to figure it out.