it would be far cheaper if you actually bought the likes of that unit outright instead of trying to make it.
but, if your not too pushed on how it'd look overall and dont mind alot of sculpting and sanding, you can
always try making it out of fibreglass. the only catch is, is that you need alot of materials and it can be costly.
plus, if your not used to working with fibreglass its definately going to get messy.
try checking out the 12volt.com , they do alot of car related stuff and will have topics covering the use of glassfibre.
look at some of the ice sections / car audio sections. you will see people making sub boxes and door pods out of the stuff.
other than that , i'd just buy the product you linked to
