Find out what thread pitch the front screw is. Go to a hardware store and get a machine screw or whatever in that pitch, cut off the head with a hacksaw or a dremel and either file or dremel to shape up a rough front site. You'll need a small vice or patience and a pair of vice grips.
Just screw in the screw till it's tight, then use a sharpie to mark what's going to be the flat sides of the post before you cut off the head and flatten the sides as it'll make it easier when you install it if it's close to fitting snug and the sight post lining up the right direction.
Once you get it to the height and width you need just paint it up with some model paint and a small brush.
If it's a weird thread pitch call the local fastenal to you, they carry all kinds of weird size stuff for industrial repairs, a small piece of threaded rod would do as well.