OK, so this is a bit overkill, but it's hard to describe and it seems like the penthouse bed layout is a common question. So here goes...
Here are the measurements from my RB50 (looking down on the penthouse bed):
There is quite a bit of extra room for elbows and heads beyond the mattress in the rear area. This unpadded area beyond the rear mattress is just fine for pillows:
I use "eggcrate" foam for both mattress sections, but I only store the piece I've cut out for the front mattress section on the bed itself. I find that there is
no room to store anything above the rear mattress section when the top is down -- so I store the rear piece of foam somewhere else. If I do try storing anything above the rear mattress section, the top doesn't completely close. The mattress sections themselves are 2.75" thick, of which 1" is wood (leaving 1.75" of built-in padding).
There is more room (an extra 1.5") above the front mattress section because those two rectangular penthouse roof support pieces don't hit it like they do for the rear mattress section. (See the white padded rectangle with a metal handle on the roof in the photos below.) Here's what the front mattress section looks like with the top up & down:
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