Ok, got some problems with this model, open surfaces. Trying to create a solid

Hi, I'm trying to create a fill surface to create a solid shape from these surfaces. I then want to shell it to make a plastic part, add ribs etc.

I am having a load of problems, but the main ones are coming from not being able to fill the surfaces. I am doing a check and it is saying that there are a few open surfaces, but I have no idea how to fix them. I believe that the problem is coming from the offset surface.

How do I fix these errors?

Any help would be fantastic.

Thanks, Will.

SolidworksSurfacing