Ahh, that's cool mate, just glad to help answer the question, i'll move it to technical later.
Ok, well, there's the cheap way if you want to get your hands dirty or going the shop built route if your not that comfortable with the welder?
Mine consists of a 2" x 2 metre section of mild steel pipe that's welded to a Peco Stainless Peugeot 306 DIesel Back box! haha, The pipe was £20 and the Backbox was free so Not really breaking the bank but Works perfectly for me, As it's for a diesel it's tuned for a deeper bass note and sounds a bit like a small V8 at tick over then changes to a a full straight six wail at 3000 to full revs.
As for making one it's an absolute piece of piss as far as bends
go, We just slit and welded mine in about 2 places to get the bends as it's hardly any angle if you just run straight pipe from downpipe. Putting a flexi connector is a good idea too if you encounter speed humps and the like, plus makes it easier if you ever need to take yours off.
The reason i stayed with stock 2" is that now i removed the restriction of the center box, the gasses can flow faster through the system at mid to high revs Now - BUT
On an N/A engine like the 5MGE There is always a trade off on Flow rate and diameter V.S's Back pressure as unlike a turbo equipped engine the Back pressure in a much needed part of N/a Engine tuning.
Having an exhaust that provides Sufficient back pressure at low revs and having a nice fast flow rate at high revs is the best compromise you can have as back pressure provides the engine with low end torque at lower revs. This means there is a slight build up of exhaust gasses which provide an invisible wall for which the engne must push against from the manifold to the down pipe and through to the Backbox.
For example Bmw made a System on the E36 328 cars that had a spring loaded flap in 1 of their 2 backbox tail pipes that stayed shut at low revs to create torque, Then opened up once the engine was singing to help the flow needed to produce the Bhp.
On rolling road comparrisons with aftermarket exhausts the Original design was very hard to beat showing how well designed the original actually was!
So, to round up your question Mick, 3" straight through is about the minimum you would use on say a 7mgte / 1-2 gtze engine due to the turbo providing the back pressure, so everything after should be as short/ free flowing as poss.
On a N/a engine like a 5mge you want to keep some restriction in the system either by keeping the pipe diameter near standard and running a small backbox if you want noise, or say a small mid box like a cherry bomb style if you want to keep it a bit more civilized.
you'll probably regain more torque by running 2 boxes in the system, but will lose out on BHP at the top end if it's too restrictive, your choice and it's a bit of a black art really.
& Our cars are way before uk cat tests, so not needed to pass Mot regs. Being that they don't need to test it, within reason you can make it how you want.
My lambda sensor hole is plugged up on the manifold with a plate, and never found a plug for one in the loom so i very much doubt U.k cars came equipped with a cat, We were just about to switch from 4 star to unleaded at the time (1986) and later cars like the Mk3 had loads of Cat's built into their systems.
On an N/A engine like the 5MGE There is always a trade off on Flow rate and diameter V.S's Back pressure as unlike a turbo equipped engine the Back pressure in a much needed part of N/a Engine tuning. 
