I cant quite remember know but was a professionally made shelf and i think i paid around 80 quid for it. Gave a nice mid bass which you dont get with the speakers in the standard locations.

Unfortunately I sold the shelf when i stripped the car for parts.

Try ebay or here:
http://www.dvbcaraudio.co.uk/-c-94_101.html

cant see a sierra item listed but prob best to give them a ring.

this one looks good:
http://www.audioinmotion.co.uk/store...w.asp?item=318

Have a search on google mate as most competent car audio places will be able to do these, or alternatively speak to BassBoy on here as he is very good and has an awesome build thread on a Nissan Previa