Select a point at each corner of the map. Then hit C. That should create a ceiling layer.

There's also a handy little command-line bitmap converter here: http://ace-spades.com/forums/topic.php?id=1023#post-10966

It will take a bitmap and create a flat map out of the bitmap at the height you specify. There's also a heightmap program in there as well.

Thanks man, numbered cursors ay?

I may not use your sand map but I might take your idea of laying a texture over the map, the only command I can see that looks relevant though is P project texture, which does some other crazy thing

numbered cursors are good for that because you can move them around and repeat actions, read voxedhlp.txt

if you're lazy here's a flat 512x1 sand-like texture map you can start with:

pic:
http://aloha.pk/files/aos/images/sand512x1.jpg

download:
http://aloha.pk/files/aos/maps/sand512x1.vxl

Is there any way to use other shapes as brushes? For instance if I have an empty map and want to cover a large area 1 voxel up from the water level, the only way I can think of to do this at the moment is to spend 20 years using the spray tool. Is it possible to create a large 1 voxel high square as a brush to use?