I agree with others; glass and simple HOB. Your QT is going to get ich, flukes, algae, cyano, everything you can imagine and at times out of control. You want to be able to really easily go to town with a razor blade and disinfect every part of it. Also, flat sides are easier to scrape than curved ones.
Do what you want of course, you’re the one that has to live with it, but that’s my 2 cents after doing strict QT on everything for years.
For my fish QT I found a 20g kit on sale at Petsmart and I’m glad I went with a version that has a lid and light. I upgraded the filter to a tidal 75 for extra flow and media space. The stock heater died after a year. But, on the whole, I’m happy with my $50 purchase before upgrades. 20g won’t do it for a 6” tang, but a 40g would have been a lot more to take care of in terms of water changes and salt over the years as I do 100% water changes not infrequently.