What materials would work for a waffle iron?

Aluminum is easy to CNC mill and it can be make non-stick

It needs to be finished to a semi-gloss, and have a slightly undulating (smooth wave pattern) surface to allow for release

It should be hard-anodized to reduce surface pores and future oxidation. Do not scrub hard-anodized surfaces, just soak

Cast aluminum will most likely have too many surface pores to me useful

Believe you would be best staying with cast iron, there are machinable cast irons available, found this article, which, while looked at from a production standpoint, gives out standard sizes and verifies machineability:

http://www.productionmachining.com/articles/a-look-at-today39s-cast-iron

In your question there is nothing said about the heat source you intend to use. Believe your implied intentions are to use electric elements.

There isn't any reason not to use mild steel, although as a more finely refined product, it is probably more likely to corrode and pit.

Aluminum would not be my first or second choice for a number of reasons:

The coefficient of expansion is more than double for aluminum (6e-6/13e-6 inches/degree F), leading to the belief that warping would be more likely in the lighter aluminum.

The specific heat ratio of iron to aluminum is .11/.21 (Btu/lb F) (kcal/kg C), water being 1.0. This measure indicates the amount heat it takes to raise the temperature, iron being almost half for aluminum.

Iron doesn't fair well against aluminum in conduction where the ratio is 8/25, meaning aluminum transfers heat 3 times faster than iron.

I just don't like aluminum, maybe this goes back to the days when we used to make hydrogen balloons by dropping pellets of aluminum foil into a coke bottle with lye and water in it an putting a balloon over the bottle to fill it with gas. There are lots of examples of strong detergents really messing up aluminum cookware. I ruined a motorcycle blackout light I found in a dump one time by immersing it in an industrial lye solution. Another reason I don't like aluminum can be found at this link:

http://alzheimers.org.uk/site/scripts/documents_info.php?documentID=99

Inconclusive doesn't mean safe. I don't own or use any aluminum products which pertain to cooking or consuming food.

There are other non ferrous metallic materials such as copper, ceramic is probably not such a good idea, gold might work for the person who had everything, silver would probably tarnish and need to be polished now and then.

Hope this helps.

The problem I see with cast iron is that you'd almost have to get it cast in the proper shape. Cast iron is very brittle and would be very difficult to shape after casting without breaking, I assume. Haven't done any metalwork with cast iron, so I'm talking off the top of my head. Of course, cast iron would need seasoning as well as it rusts fairly quickly.

Steel shouldn't pose any problem, if it's properly seasoned. I don't know about aluminium, for some reason this is a metal I don't want anywhere near my food ever since I tried to whip cream in an aluminium bowl... It has the advantage of transmitting heat better than steel, but I'd make sure to have it coated with something.