Make a bed in a Toyota Starlet?

What is the most easy way to make a bed in a Toyota Starlet EP91 (my "new" car)?

This is my testing..

Pretty flat, but with some bumps..

The box is to low.. 26cm in height (I think).

Trying some other box, a fridge container for example

Looking for the perfect box size: Max length: 60 cm, around 50 is best.
Height: 36-37 cm
Width: max 40 cm
I think Hammarplast Smartstore Robust 45 liters could be a good size (59 x 39 x 33 cm).

For me to remember...

width of box: ~35 cm

Max lenght: ~59-60cm

The wood-board can be moved around.

Here the wood is on top of the plastic boxes.

Me trying the "bedframe"

Me again, ..

I plan on using this for heat transfer to a isolated box containing PET bottles with water. Heat storage

Picture of bootarea and rib

Bed-size, the cardboard is about 62 cm wide. Total Length is about 190 cm. So I will try to find a suitable mattress, about 80 cm wide and about 190-200 cm in length.