I actually tried a couple of solutions. First there is the official case from Radxa;
It has enough room for the PoE HAT and M.2 SSDs on the bottom. Depending on your use case, it can cause the 5B+ to run a little hot, even with the PoE fan running at top speed. My solution for that was to just drill some holes in the top to help vent the heat. If you are so inclined, one could probably mount a fan on top for an even bigger boost.
I have also used this KKSB case too:
Not technically for the 5B+, so no every little thing lines up (like HDMI input is completely covered) but it does fit in it. The PoE HAT is just a centimeter or two too tall to fit the top on (after removing the heatsink). If there are some extra screws and nuts laying around, one can create a spacer jig to raise it up and still attach it. Or just run it topless.
I’d try the official one first though and just mod it if needed. It’s way easier than make a small spacer to raise the roof, so to speak. Hope this helps!