According to the info I have on hand currently,
a Sprinter's interior length states: 121" or 161" or 177".
So as bigger is always better in C/V Expediting... take
177"-156"(4'x3=12', plus 12" extra) = 21"
So it might look like 21" is all that is left for a
"fixed type sleeper"...
Basing this all this on a "standard 4x4 skid size with just a bit extra.
Refer to the other thread "Sprinter Sleepers" for some flexible suggestions others are using. Comfort costs space and space costs money, how much can you afford to have or lose?
Just saying...