US Army tartan, huh? Well I would think there are several options for a cap badge:

1) enlisted cap badge with branch of service colour behind it. (picked up at a gun show or surplus store for around a buck)

2) branch of service devise on a cockade of the colours (crossed rifles on inf. blue; crossed sabers on yellow; etc.)

3) your own clan badge with either your distinctive unit devise or branch devise placed alongside (or below)

4) unit shoulder patch stitched on (big red one, sheaf flaming sword, etc)

None of these should set you back more than about $5 or $6