Here is how I clamp up my boxes. I use blue painters tape. Line up your stock, I use a straight edge to keep everything flush. The edges should be touching but not overlapping. Give the tape a little stretch and press onto the stock. Don’t tape the ends yet.
Then flip the unit over
Apply your favorite stickum. I use Titebond and like to pre-glue. That is, I apply a very thin coat to all mating surfaces. Then turn around and apply another coat. End grain soaks the glue up pretty quickly and the pre-glue, I think stops glue starvation. I don’t tape the ends before I glue as I usually slop glue on the tape and that gets messy.
Now just stand up the left edge. Insert your panel into the created ‘L’. Grasp the far right box side and carefully stand it up guiding the panel into the dado/groove. Now just flip the remaining end over and seat it onto the box. With the untaped side away from you support the box with your chest and press the joint together. Having tape precut for this step is helpful. Press/stretch tape over the joint. Check for square. If your panel is square this will help immensely.
If you cut em right they come out very nice.
Next step is to cut miter keys.
-- Made in America, with American made tools....Shopsmith