ideas on attaching front facia on my box

You can screw "ears" onto the sides of the box (made out of 2 x 4 or similar) and this will give you some "meat" to support the sides of the cover.

as far as fastening you can use screws with a choice of "dress up" caps or countersinking. If you want it totally clean but removable I sometimes use "hook and loop" strips which I staple on..kind of like what is done to secure some home speaker grills. It is alot stronger then velcro and if you staple it the glue not sticking well is a non issue. If you find the cover is to thin to staple to you can use an epoxy or CA glue to support the hook and loop for strength.

below is simple diagram of the "ears" screwed to the sides of the sub enclosure. In addition you can make the "ears" any shape you want or need depending. Or you can use MDF cut to shape(s) and screw those to the "ears" so you have many ways to make a solid surface for the cover to be sturdy.

Good Luck