Flat ironing is not permanent, and any moisture will make the straightness go away, and getting your hair wet is a surefire way to lose your hour’s worth of work. Since I also have really thick hair that takes forever to straighten, I usually keep it flat for a few days by not washing it at all. I tie my hair in a high bun, do my best not to wet it in the shower, and use a straightening iron to fix up the little wisps after the shower. Thick hair is great in the way that it’s actually healthier for it not to be washed every day, so you’re in luck. The natural oils in your hair actually make it look much nicer after a day or so without washing. As long as you’re not using crazy amounts of product in your hair every day, you should be fine without washing it every day. I’ve also found this video which has some tips on keeping your hair looking nice and non-oily in between washes. Good luck!