I'm a big fan of Walter's elegant one-piece cover for rpi3. However I found out the cover does not fit rpi3b+ because the revised design features a slightly thicker PCB. (If you force the board in, the cover will be bent upward and not sit flush at the ports; see grey cover in picture #2.) I made the cover thicker by 0.6mm and now 3b+ fits perfectly. It's a minor revision and all credits go back to the original designer.