we propped 2 of our boys (Gibby preferred to lay flat)... we had a board under their mattress - b/c we use angelcare monitors and it helps them work better... but it also helps elevate easily - we'd put the rolled up towels under the board, so the mattress was not "bent" but was at a true incline.
I have 'wedges' that I bought at BRU... I have one in each of their cribs.... and I also bought 2 wedges for my double/twin bassinet... DH trimmed them to fit each side of the bassinet.... I then cover the wedge in a pillowcase.
I also bought wedge #5...and that wedge is in the PNP...sideways....so both girls can nap side by side.... it has worked well so far. (I also put this wedge in a pillowcase and they lay directly on it.)