Sure, this is a very competent design but i dont think taht problem exclusively apply to thin plywood boxes with straight edges, one example is earl geddes speakers. Generally very good directivity and large roundovers on all edges but the 0° axis (not the listening axis) is a little ragged. According to him that is a concious compromise to get the best sound where you listen. The speakers are toed in 20 or 18° wich will give you a wider sweet "spot" due to time intensity trading and balances out the a little bright 0° axis when you are at that axis from one of the speakers with the weaker off axis from the other. If you would EQ the 0° axis perfectly flat all other would be crappy
Geddes had a post about it on DIYAudio that I would dig up were I not traveling right now.