Yip - posted a little while back about this -link here I've an all black screen wall (located some extremely "flat" black velvet - it really eats the light) and have screen masks that slide into place - they're connected to each other, so when you move the top mask, the lower mask moves by the same amout at the same time.
I blush everytime i get asked for pictures...... everytime i go to buy a digital cam i get distracted and buy another amp or something! For now, the best i can do is a description.
The screen is DIY as per many described here. I used 4 lengths of 3 inch by inch (ish) wood cut to length. Instead of squaring it before i put it on the wall, i put it up and packed it out with scrap bits of wood so that it was flat and square, then bolted it to the wall. From there i stretch blackout material over and painted. The borders for the main screen are simply smaller strips of wood wrapped in black material and nailed around the edge of the screen. For the addition, i got another length of the smaller wood and wrapped in the black material again. However, instead of finishing it and cutting off the black material, i left a large amount of material free. From there i worked out where the top border for my 2.35 ratio screen should be then cut the material at a length so that when the wood was allowed to hang, it hung at that top border position.
So when i want 16:9 film, i have this material and wood rolled up and set above the screen, for 21:9 i then roll down my extension and it blacks of the top of the screen accordingly. Much simpler than the pulley systems i have seen some using, but it does the trick, cost me nothing and took about 2 hours to do