I also use a Jasper jig when cutting openings with a plunge router.
For the carcass (sides, top, ans bottom), I cut the sides to the exact dimensions, 10.5" x 12". The tops and bottoms are cut to exact width, 10.5" but I cut the width slightly oversize at 8.625". This is to allow about 1/16" overhang of the sides. Once the carcass is glued up, I use a flush trim bit to make the edges of the tops and bottoms flush with the sides.
With butt joints, I use corner clamps and cut spacers that are 7-1/2" x about 3" to keep the parts square and the inside opening correct at 7-1/2". You can use brad nails to help hold the pieces in position before applying clamps.
Butt joints are the easiest to cut but locking rabbet joints, while a little more complicated to cut, make glueup and keeping things square easier. This joint is sometimes referred to a quarter, quarter, quarter or half,half,half joint. As mentioned, YouTube has a number of videos on how to cut this joint. Stumpy Nubs is one that comes to mind. If you go with locking rabbet joints for the carcass, DO NOT cut the top and bottom pieces over size. All four sides of the box must be cut to their actual size or the inside dimension will be off.
For the baffles, I cut all four sided oversize by 1/8" - 1/4". This allows a little bit of adjustment room in case the boxes don't come out completely square. I glue these on as butt joints and use a flushtrim bit to smooth everything out once the glue dries.
The reveal already mentioned is a good touch.
Mike