Hi Jack - one possibility is to take the 2 longer curves and split them up so that they are in 2 pieces.

If they are intersecting each other you can do that by selecting them, run Edit/Trim, then push Done (to indicate that they will cut each other), then Done again (to indicate to keep all pieces).

Once those are split into pieces they will form a network which can be surfaced by selecting all the curves and running Construct / Network.

If you need more help, could you please post the .3dm model file and your image backgrounds? That would make it easier for me to try different things.

- Michael