It's hard to see in the picture, due to the color I used, but there will be a ledge all the way around the rails, so the bed will be supported all the way around, in addition to the cross support in the middle.
That ledge will be on top of the ash, so the mattress doesn't rest directly on the part you've seen so far.
Code:
..MMMMMM
CCMMMMMM M = Mattress
CRMMMMMM C = Corner (the part I've been making)
CRMMMMMM R = Rail, including ledge
CRMMMMMM F = Foot
CRRR
CCCCCCCC
.....F
.....F