I used a piece of the corregated plastic that is used for signs. It is stiff enough to stand upright. I used a compass, as described above to pencil a curve on the sheet. Sign shops may have scraps that would be cheap or free.
I found small, but noticeable differences between curb and road side of the trailer. I used braid from a hobby shop to trim where my new cabinets meet the wall.
John W. Irwin
2005 Classic 28 "Sabre-Dog III"
2013 Silverado 2500HD Duramax/Allison LTZ
WBCCI #9632, TAC TX-10