Nortons Idea with the concrete rings is a good one.
I have spend most of my professional career as a concrete specialist for large commercial projects , mostly High Rise construction in NYC.
Definitely not Q con , they are very moisture permeable , not a good attribute in the construction of water containment structures.
when I did mine I did a monolithic (i will explain soon) pour with a water resistant additive to the concrete, (It's been a few years and I don't remember the name of it in Thailand, I am sure one of the other members will remind me)
-Smooth the bottom ground,
-Make the outside form box , if you are going to use the form repeatedly, use plastic faced 5/8" ply wood, plain plywood is fine for one time use.
-lay your wire mesh on the bottom and sides making sure there is at least 1" concrete coverage all around it.
-make the inside form box 4" shorter than the outside form box , you need to "chase " the inside box so that you can srip it after the concrete dries, especially if you will re-use the form. (I will make sketch below)
-drop the inside form inside the outside form , hanging it from the top, leaving a 4" gap at the bottom.
-place drain , and feed PVC pipes in the form
-Pour the 4" inch floor and wait a few minutes until it stiffens enough that when you pour the wall. the weight of the wall concrete does not pour in the floor, but not so long that you have a cold joint between the floor and the walls.
-Now that you have the floor poured you can place a couple of braces inside the box from one wall to the opposite wall to keep the walls from kicking in.
(if you had done that earlier it would had interfered with the floor pour)
-Make sure you vibrate the concrete well, but not so much that the aggregate settles to the bottom. If you don't have a concrete vibrator, thake reciprocating saw , remove the blade, place the shoe of the saw against the form, you will be amazed how well the reciprocating action of the saw vibrates the form.
The sketch is not o scale ,use only as example. I colored the onside box to display the "chase" (one edge inside, one edge outside ) to alowe easy striping
Cut out to show inside box hanging from the top and 4" of the floor (when pouring the floor it is ok to let it floe on the walls but dont let it higher than the inside form so you don't lock it in.
It sound more complicated than it is, it is really very easy, a couple of hours of work.