I've been doing alot of research on this lately too (Working towards setting up my fist saltwater tank...for like the last month... I'm slow). QT tanks were on the top of my research list.
Alot of what I read suggests just using good sized PVC
piping (And a bit of live rock), and not even worrying about substrate and such. Myself, I'm kind of planning a "mini-fOWLR
" which will likely end up living in the middle of my dining room table. My logic here is A: I rent, and don't have alot of space and B: Means I can keep a close eye on the blighter I just bought so i can head things out. Basically, even my quarantine tanks are way underpopulated display tanks.
Esentially, it's about as decorated or undecorated as you like, though anything (Like live rock) you put into it might be destroyed by meds or anything else you have to do if things go wrong, but ten pounds of live rock is better than thirty, fifty, or a hundred (And that's ignoring the FISH, inverts, and everything the heck else.)