I bought a 10ft long Desert/Arid back scenes (from the New Modellers Shop, see http://www.newmodellersshop.co.uk/backscenes.htm). I cut up some 3mm MDF in 8inch widths to use as additional back scene boards, these rest reasonably well again the fixed ones and can be held in place with blue tack if needed. The back scene papers were glued to the boards and the excess trimmed off.
For better side back scene boards I re-used my new sky ones (See http://adventures-in-the-indies.blogspot.co.uk/2018/02/alternative-table-back-scene-boards.html ), along with the best bits of the offcuts and stuck them to some stiff card, these rest nicely against the plain sky side boards.
It all looks quite effective with my sandy-ish coloured felt game cloth and limited amount of desert/arid scenic items.
next bits to do are some more sandy lumps and bumps; and figure out the best way to represent sand dunes.