Hi Tony, do you mean you want to make a surface to fill in the thin amount of empty space between these 2 pieces?

One method you could use is Loft, you would select 2 edges like this:



Then run Construct > Loft to build a loft surface between those edges:



Then select the resulting surface and the other original pieces and use Edit > Join to glue them together.

If you want a smooth surface between them you could try using Construct > Blend instead but you probably will need to open up a little more space between them to give room for the blend to be shaped.

- Michael