what do you need?

one sheet of NEPOPREN* - 3-4mm,
(min. 50*70 cm)
a sharp cutter,
glue (for plastic - "contact glue"),
velcro "tape" (Klettband).

*
(rubber foam - EVA foam - "Moosgummi" /
online ressource: Germany - MODULOR http://www.modulor.de/ )

how do I make it?

look at the plan.
cut, fold and glue it together.

folding tip

slightly/carefully warm on the (white) lines where you want to fold the sheet with an iron.
(test first with a little piece of NEOPREN*)