Hi Stefano,

re:
> Can someone please let me know, how do i fill this shape into a solid...

The general technique is above in this same thread:
http://moi3d.com/forum/index.php?webtag=MOI&msg=3723.12

For your case here to start with the curves need to be cleaned up so the pieces meet up exactly end to end and are precisely mirrored. You only need to bottom ones though, join those together into a single curve like this:



Then draw in an angled line here:



Select the line and run Construct > Sweep and pick the bottom closed curve as the rail path, that will generate these surfaces:



These are the surfaces you need but they need to be trimmed up. You can use the Edit > Trim command for that cutting them one by one but there is a shortcut you can use if your initial construction curves are clean and precise. The shortcut is that the boolean merge command can be used to extract out a center volume from a piece like this and slice away all the outer pieces in one shot. To use that you would need to put in a ground plane like this:



Now select all the surfaces and run Construct > Boolean > Merge and it can extract out the inner core solid:



3DM model file is attached.

> Also as this post is quite old any faster / alternative ways to acheive similar results ??

Using boolean merge will be quite a bit faster than trimming each individual surface. But make sure your starting curves are cleanly constructed, ends exactly touching and opposite pieces mirrored.

- Michael