Use some 3/4" CDX plywood for a bottom and cover with some exterior carpet from Home Depot or Lowes. Fasten the carpet to the wood with glue, staples or screws. I usually roll the carpet under the edges of the plywood and use screws/staples. Mount a seat base in the front half of the boat and one in the rear that is close enough to reach your motors on the transom.
Another option is to move the rear seat base forward enough to sit on the bench seat to operate the motors. That will allow you to be closer as well as having a lower center of gravity when motoring. You can put a pad on the bench for comfort on the long runs. Those square seat cushions that double as throwable life savers work great. It you will be out after dark, either buy detachable running lights (Wal-Mart usually has) or run some wiring and attach light sockets (also at Wally World). All of this should run $200 or less and can be done in a day if you have all the tools. Measuring and cutting the plywood to as close a fit as possible is time-consuming but worth the effort. Take the time to measure and plan before you start, it may save you grief in the long run. You may have to put 2x4 chocks under the plywood to support it out near the middle if there is a large gap between the floor and boat bottom. One word of caution...the flooring will add a lot of weight to the boat. It won't affect the depth the boat runs very much, but it will make it harder to load/unload by hand. Obviously, if you have a trailer, it is not an issue. Good luck.