2.25" bore, straight through silencer (make sure the bore does not reduce internally). silencer needs to be long enough (your restricted on diameter) to reduce sound levels to the requirements of the tracks your racing on. You can use the mini type mounting rubbers to attach the exhaust to the floorpan.
I have a mk3 2.0 8v too do you have a spare air mass meter by any chance? mines duff.
Ok, here's what i've got; 2.25" from manifold, no cat, no centre box. Rear box is a straight through oval unit about 18" long. I got all the 2.25" pipe, bends etc from Red Star, just rummaged in their parts bins, the box was ment for a rover of some sort........just made sure it was straight through, no reduction in internal diameter and of perforated tube construction on the inside.
All the bits cost ~£40 in total and took a day for me to fit and weld.
Although its completely straight through its quieter than some stainless systems i've heard, very liveable on a daily basis.
It won't provide any restrictions to your setup.