A simple solution I used was one of the 2 layer acrylic cases, with the SSD replacing the top. Start with this (picture is link):
And replace the top with the SSD.
This removed the top and fan, but the fan was loud, and wasn't really needed with such an open case.
I later got some additional spacers and added the top layer with a quieter fan back onto the case (SSD on top of that).
I have another SSD booted Pi 3B+ in a Flirc case with the drive in an enclosure (held in place with some Fun-Tak/Blu-Tack).
Drive in this case is a tiny mSATA form factor SSD (I'd put the Flirc on top of a normal 2.5" enclosure).
There are some ready-made solutions for a Pi and drive inside a case, but those tend to be quite large.
My mind is like a browser. 27 tabs are open, 9 aren't responding,
lots of pop-ups...and where is that annoying music coming from?