Back in '00, I built a completely new sound bar out of MDF to house the Pioneer 6x9s I had. Built it with two chambers and covered it in padded naugahyde A little time consuming to make, but the results were worth it as I was able to angle them slightly forward so the sound was not shot straight down.
I did this as I was not happy with the stock sound bar on the '97. Proceeduraly, I stretched a string across where I wanted to put the bar, took measurements and cut a card stock pattern to check the fitment prior to committing the 1/2" MDF. Added sheet metal to the ends of the bar and screwed the thing to the top of the Heep which required removing the trim to do.
Took maybe a couple of days to get done. Not difficult, just time consuming...