Hi, when you say you want to build some studio monitors, what do you mean. Do you plan to mix music on them. If so you really need to take measurements and design a crossover that will give you a flat un coloured response. This is a significant amount of work and will almost certainly require several iterations.
If you just want to make some cheap speakers to play back instruments then you approach will create a speaker that will play back sound but it is very unlikely to be flat.
For a monitor speaker if you haven't designed a speaker befor I would recommend finding a design that some one has already done. You can then learn from their design and end up with a speaker that will be suitable for your application.
Zaph creates very nutral designs that would probably be suitable for monitoring.
He also gives a good description of the design decisions so you can learn.
Zaph|Audio
Regards