Measure the inner frame of the area where you want to add slanted slats.
Cut your timber slats the length you require, based on your inner frame measurements. In addition to the space of the opening they'll fit, allow a 1/4-inch at either end to slot into the 45-degree angled grooves that will hold them in place.
Round the four long edges of each slat. Set up the router using a 1/8-inch radius roundover bit with a bearing tip, and adjust the depth so that the roundover is flush with the router table top. Move each slat along the bearing from left to right, so that it moves against the rotation of the roundover bit.
Sand all the slats. Use fine sandpaper so that no sanding marks are visible, whether you paint the slats, stain them or leave them bare. Your slats are now ready for installation in grooves that have been cut or routed out at a 45-degree angle.