Cast aluminum will be heavier than steel in most cases being that the rail castings and or cooling/ornamental fins on top are usually thick. They will eventually crack though since it's a casting.
Stampings or welded types are better, but.........
Man, funny enough you must have missed my say in what I've done with mine.
My covers believe it or not are the infamous thin stamped Gold or Blue aluminum anodized MOROSO covers that everyone hates.
Direct Connection also had them made.
If you look back to who knows where on this long thread, I carefully body worked the MOROSO logo flat with body hammers and a solid dolly as support from behind then went lightly in with a disc sander to get all the logo evidence off and then lightly putty and sand for a smooth stock STEEL look. (Careful with this procedure because one can easily cut through the covers skin with an aggressive disc and or pressure.)
413 Decals went on for more of a stealth look.
End results: They weigh about 2-1/2 lbs all together compared to 5-1/2 Lbs for stamped steel. Though I have not weighed them as of lately, I presume the Cast aluminum covers probably weigh in the park of 10 lbs for both.
Now mind you, I made sure that the rails were straight in the first place and used good sealing MOROSO blue gaskets and aluminum studs with (Important) aluminum lock washers and nuts to make a good non-leaking seal without having to tighten too much. Tightening these types of covers is what makes them leak hence the hatred towards them, but are Ultra lightweight.
Use of pressure spreaders on the mounting nuts keeps 'em from warping out of shape as well.