As far as 4 wheelers go this is what I can tell you. HU are pretty much useless unless your customer really really wants one. In this digital world it is all Ipod/mp3. No skipping perfect signal everytime and they make plenty of really cheap mp3 if they are worried about ruining their Ipod. Ill tell you what I do for several friends/customers. It comes out very cheap and there are ppl that sell the exact same setup online for several hundred.
You will have to get your hands on a very very small amplifier. I tend to use those that are made for motorcycles stereos. Very tiny almost "in line style" 2 channel running off rca to 3.5 jack cable using mp3/ipod for volume control. You can pick these little things up on ebay and like sites for anywhere from $12 and up. They are just cheap little things they will run a good set of 6.5". If you just search ebay for "motorcycle amp" you will get several pages. You will see the reason for the small size in a second.
There letters are the key here "PVC"
Im not sure the exact size or # rating of the pvc I use but on the flared out ends a 6.5 marine speaker will fit in it just perfect. So.... Get a strait pvc piece as long as you want the set up to be wide. On the ends of this use 90 degree angle pieces...use the good pvc glue...mount speakers right in the open ends of the 90s, strap or bolt to front rack and you are good to go....well running wires.
I generally put the small amp right inside the strait PVC piece with double tape....drill hole in bottom for wires. I use a toggle for on/off because my friends/customers tend to like to use them with the key off. I have put the toggle in the PVC but what I really like to do is mount a 3.5 jack and a lighted switch right in the key area. Running a 3.5 jack allows you to hide all wire instead of just having a cable hanging out.
You can get really creative with the PVC it is really fun because it is so cheap to build. I have used 90s w/ Ys to have 4 6.5s and just anything you can come up with.
This is a good CHEAP sealed solution and actually looks and sounds very good. You can paint the PVC or put stickers or vinyl designs on it...dont forget you can sand PVC!!!
I have also done alot of fiberglass enclosers w/ and w/o HU. They are just not cost effective, time effective and they require several layers of matte to withstand the vibrations!