Hi Graham, the main part that you're missing there is that when you do the extrude, you need the curves that you are extruding to form a closed loop so that it will create a fully formed solid with a cap on top.

The example I gave before used one single larger closed outer loop and then divided the result into smaller pieces. That's kind of convenient if possible, but sometimes you may want to form smaller closed loops in areas where one larger outer loop is not so immediately apparent.

But the main thing is the curves that you extrude should be a closed loop.

Here are some step by step details on how to proceed on one portion of your curves.

Let's focus on this region outlined in red:



I've selected a couple of the curves that bound this region - the problem is that the curves are too long, you need to slice up these curves so that you can pick a set of curves that only outline just this region and not a longer part.

You can slice up a curve by using Edit / Trim. So in this case, select the 2 curves shown above, and then run Edit / Trim. MoI will show a prompt in the upper-right corner of the screen that says "Select cutting objects". Select the upper line of the region and push "Done". Next MoI will show the sliced up results and the prompt says "Select pieces to remove or push Done to keep all". In this case we want to keep all pieces, so just push done again. The result is that those 2 longer curves have been cut at the intersection point with the other one, so now you have smaller pieces that can be selected like this:



and then extruded, and since this is a closed loop the extrusion will get created as a fully closed solid with end caps in place. This is the key thing - extrusion of a bunch of curves that don't touch each other end to end will not create a solid, it will create only the surface side walls. Sometimes that is useful, but booleans are easier to understand and more predictable when you do them on a solid object rather than on a non-closed surface object. So I'd recommend that you only extrude closed loops so you can work with solids.

Then you would repeat this trimming process on your other various curves, dicing things up so that you can create closed regions and extrude those one by one.

Let me know if you need more details!

- Michael