I’m not sure if this was a design decision or not, but I haven’t been able to build a makeshift bed underground. I’ve only tried to build one underground at the evac shelter (I did search for other places to build one underground, but I couldn’t find any underground places where I started!) but wasn’t able too, yet I could build one in the evac shelter above ground.
It’s an oversight due to me changing basements from regular floors to stone floors. The same will happen if you try and build a bed in a laboratory, I’ll work something out for the next release, but it’s definitely not the intended behaviour. (Cots and rollmats work everywhere due to being traps, but the bed itself is an actual terrain tile, the reason it requires a floor is because when smashed it turns into a floor tile)