Actually from the screen dimensions shown (11.25" x 15") that is cut for a 19" 4:3 monitor. ( FYI == Using the Pythagorean theorem you would have learned in HS the diagonal of the screen would fit a^2 + b^2 = C^2 or 11.25^ + 15^2 = approx 19^2 so the bezel is cut for a 4:3 format 19" diagonal monitor) What type of monitor are you planning on using ? (widescreen 16:9, 16:10 or 4:3) and how big of a bezel does the monitor have if leaving it in the case ?? (as that will make a difference in the dimensions needed.)
Mainly to adjust you would need to know what the dimensions of the monitor are and adjust the width and possibly height of the cab so that the monitor would fit. If using a widescreen then it may be adjusted by just increasing all of the 20" cuts (width of the interior) but if using a 4:3 monitor it might also need the height adjusted which would mean adjusting the dimensions of the side panels as well.