Unless you want to drop some serious cash on LED corners, youi may want to go with a strobe/LED hyrbid. Keep the four corner strobes and make the rest of the bar LED. 400 series LEDs can be had for not a lot of money, especially of you get the GEN I lightheads. You can also use 700 series. You will need some of the plastic mounts that the Edge bars use for the strobes and halogen light heads. You may also need a flasher for the LEDs.
As for filler plates, go to Lowes and get a set of beige Levolor vetical blinds. They are about $11.00. If you cut the blinds down by about a quarter of an inch and then cut to the desired length it will slide right into an Edge frame. You can leave it beige or paint them black. You can also paint them in hammertone silver if you want them to look like aluminum, especially if you go with clear lenses.
If you get the bar down to just the four corner strobes and the rest as LEDs you can power the whole thing off of a cigaretter lighter. In fact, I get cigarette lighter cord by the spool at Radio Shack and use it for lightbar power cable all the time.
Depending on the number of heads and how fancy you want the bar to be, you can use a ULF44 flasher or one of the less expensive Sho Me models. If you get your hands on a cheap LED arrow stick, you can mount one of those inside the back of the frame between the corner strobes.
You can also cut some flat aluminum stock and screw it to the plastic light head mounts. Paint it black and then it gives you a nice flat surface for mounting all kinds of surface mount light heads; TIR3, TIR4, LIN6, Pimp Pods, etc.
You just have to get creative!