angle and bracing in MDF

Bracing is not visible, so you can get by with using shortcuts as long as the result is stability.  To attach an angled side to another side that is not parallel, you can use cleats on those sides to attach the brace to.  A cleat is just a length of MDF that is used for this purpose;  it can be of any size that will secure the bracing.  A strong glue, like Gorilla glue that has a 4 hour cure time, is a good way to ensure that there is no movement in the bracing.  Here's an illustration I whipped up:

By using cleats, you don't have to worry about getting the angles on the ends of the braces the same as the angles of the box baffles.  And bracing can be accomplished using a wide variety of methods, so don't worry about having to follow a strict set of rules.  You generally want to brace the most susceptible baffle, the face, to the back of the box.  Also brace any large expanse of baffle whether it's sides, top or bottom, as these larger expanses are more prone to flexing than a smaller baffle.  Here's something I drew one time to demonstrate the weakest areas of a sub enclosure: