Making a proper FRFR cab involves a fair amount of science; you don't just throw together a few parts. Please spend some time researching the subject. The drivers need to be matched to the cab dimensions, etc. If you take the cheap route, you will get cheap results!
The other question is what amp you'll be using it with - it it's a Rockman you likely don't need an FRFR cab; a good 3-way design will work. If that is your direction, picking up a used set of PA speakers might be the best option in terms of time and money.