Found this case for the Raspberry Pi 3 Model B and the 3.5″ TFT LCD. Got it from Adafruit, where I tend to look first for any accessories, but you can also buy it directly from Pimoroni (they make similar cases for other Pi models or if you don’t have a connected display). The fact that it’s blue (hard to see from this photo) helped make it a quick purchase.
The case is a really neat design, using 10 different layers (there are also 2 extras depending on your display and Pi model) that fit around the board and all of the ports. The 4 plastic screws don’t even go through the Pi board itself at all, which you may think wouldn’t be secure. The way the different layers fit together around the ports means nothing moves even if you shake it. It’s snug. Here’s also a close shot of the clear bottom.