a sub box like the one in the link can be made for around £30, i know coz i made one
a sheet of 12mm mdf £8 local diy shop
spray glue £5 (ebay)
sub box carpet around £10
speaker connections, like in the link, £5ish
a few screws n wood glue.
plus a round 2 hours to make it, think i did rush some of it as the shape could have a better fit but does it's job.
built it around 2 years ago but never got around to having the rest of my ICE re-fitted from my last car