You do not need anything except the wood shaped into a rounded sharp edge. The edge is simply to apply localised pressure and a bit of wood does that. Slipperiness is achieved by spreading the scotch glue on the surface of the veneer (a couple of dollops usually suffice). If you use a saucepan with hot water, a baked bean can for the glue and the spare space in the saucepan to put your hammer when not using, it, the hot water will melt any hardened glue and keep your hammer clean.
I usually make a new hammer from scraps for veneering with scotch glue - takes a couple of minutes and as I don't hammer veneer that often, the extra time doesn't bother me.
I'm not sure why you'd want to make one when you can buy them from the Art Veneers Company for £8.95 (6cm (2 ½”)) and £11.95 (10cm (4”)). The Art Veneers company is at: http://www.artveneers.co.uk .
If you're laying marquetry, I can imagine aluminium struggling to cope with some of the harder veneers (such as ebony) but perhaps perspex would be more resilient. I've never tried it so I don't really know.
Chris' solution sounds quite reasonable. The problem with aluminum is that it can leave blacks marks on the veneer. Same with some steels. I just cut up an old brass hinge that I got from a flea market.