Its easy to say make one, ive got a hole load of engineers and equipment at my disposal but its not worth the hassle for the sake of £40!
Its a case of getting the measurements and making it near on perfectly true so that it seals, a fabricators going to charge you more than £40 to knock one up, and what ever you do dont try and nick the flanges off the cat cos then you have shafted you cat altogether - you will want it for MOT time ( do a search for the debates on this before everyone starts that old argument again)
If you have the option get a silenced one off Ktec, it wont sound any different to normal (thats what i have)