The brace is kept as simple as possible. It makes the enclosure more rigid and will move panel resonances up where they don’t get excited by frequencies generated by the subwoofer.
It gets sunk into the side and top/bottom panels about 5mm. The window cuts (145 x 148) don’t need to be exact, this is just an indication. I’ve cut these with a jigsaw while all other panels were cut to size on my router table.
The trough holes for the threaded rods are cut a bit wider, as they serve no other purpose for the rods.
The Side Panels feature the 3mm thickness logo panel on the outside. This makes them 19 + 3 = 21mm in thickness. The logo panel is made slightly oversized so you can glue it on the side panel and cut it to size with a copy bit (the one with a bearing on top). On the inner side it has the cut-out for the brace to fall in. It also has some holes at both ends that connects to the top and bottom panel with dowel pins (6 x 30mm).
A sample picture of the brace cut-out (using one of the baffle panels as the brace-template):
Here’s a link to the logo (zipped DWG) I used to have it lasercut: click to save.