Looking for help with installing dryer duct in an interior, 2"x4" framed wall (depth about 3-3/4"). Having trouble finding how to transition from the standard 4" diameter dryer connection, to an 90* elbow and duct run will fit in the 3-3/4" wall space. I need about a flat or oval duct piece of about 18" to get through the wall and down to the crawl space, then transition back to 4" round again. Any thoughts? Tried the pvc recessed dryer boxes that come with the transition boot, but the cheap boxes aren't rigid enough for me to use.