Well, short of using laser-cut aluminium, the next best is really Tufnol (phenolic plastic). There are two grades, paper re-inforced (cheaper) and cloth re-inforced. This is the brown stuff found on the bottoms of some routers.
Acrylic doesn't last as long as phonolic plastic, and it will craze with time, but it is plenty hard and a LOT cheaper. Offcuts can frequently be cadged from signwriters - it is used for shop signs - in thicknesses of 5 or 6mm which are ideal for templates. The only downside is that it is a pain to sand (tends to ball-up when it gets hot).
For both materials I have found it best to make an initial template in MDF or plywood and get it really accurate - then copy rout the template out of the harder plastic using a sharp bearing template cutter.
Scrit