You can directly put the hot tub above grass or concrete as long as there are no objects that can puncture the material. You can make use of a folded or cut-out tarp to serve as an extra layer of protection for the bottom part of the hot tub. But it does become slippery when it gets wet.  Rubber tiles or foam can also be good alternatives; you can buy them online or from physical stores. These can be good protection for your hot tub from sharp objects underneath. But if you really want a long-term and more reliable solution, a hot tub pad is your best option because it also serves as insulation.