When the occasion arose and I did not have a table saw for repeated accurate cuts, I got a piece of 3/4" plywood, in your case maybe a 2'X3' piece would do, plunge the blade of a circular saw through the plywood or better yet cut a 1"X6" hole in the middle of the plywood to set a circular saw blade through. Using fender washers and screws, clamp the saw to the plywood locking the base of the saw in tightly. If you don't mind a few holes in the base of your saw, screw directly through the base after you drill a few holes in it as close to the corners as practical. this Is what I did, they come in handy for other purposes too. As a mention, needless to say you need to govern how long the screws are so they don't go all the way through, if they do, grind them flush.
Turn the whole thing over so the saw is under the plywood and the blade is the only thing showing, just like a table saw. Set your angle, set a scrap piece of wood for a fence, adjusting it for it being parallel with the blade using scraps to test cut and check the width of your cut, then make your parts for your box.
With a little planning you may only need to change the fence 3 times or so to do the whole thing. Once you get the fist cut set up right, draw a line on the table to serve as a parallel reference to save all the tweaking for each cut.
As a caution, no there is no guard, yes you can cut your fingers off with this, same as a table saw.
Last tip, get yourself a
decent blade for this type of cut, the joinery will turn out sweet, using tape along, not across, the joints to hold it all together until the glue dries. The link is for a 24 tooth blade, some folks may say you need more teeth, I have seen some blades with 10 teeth cut satin smooth cuts, this is all you will need.