Lay followers are only expected to keep to the five precepts. Eight precepts are usually kept if you are staying in a meditation center, monastery or when you go to the temple on Poya days.
You don't modify higher precepts, you just don't take them if you are not in a convenient place to follow them. By the way, the eighth precept does not require you to sleep on the floor. It's about avoiding high and luxurious furnishings such as these — over-sized couches, couches adorned with carved animals, long-haired coverlets, multi-colored patchwork coverlets, white woolen coverlets, woolen coverlets embroidered with flowers or animal figures, stuffed quilts, coverlets with fringe, silk coverlets embroidered with gems; large woolen carpets; elephant, horse, and chariot rugs, antelope-hide rugs, deer-hide rugs; couches with awnings, couches with red cushions for the head and feet etc.