I can't offer much help since I don't live there anymore.
Building something in Sri Lanka can be the most time inefficient thing since things can go very slow and also they may try to extort you.
There are two ways of doing it.
1) Hiring a company to do everything - most expensive way and also the most efficient.
2) Partnering up with a respectable local and sub contracting everything.
This may take time and you can save some money.
I think you can visit some of the cabanas and get some idea. I have looked at some in Google.
If you want the cheapest, I'd stay with the Ancient Sri Lankan building methods of making the frames with small bamboo and filling the space with mud balls and roof with coconut leaves. You may want to put a mesh under the coconut to prevent reptiles from entering. These kind of structures are very cool inside and there is no need for AC. I have also seen structures made from wood and local tiles etc.
Getting a building permit can be a nightmare and you may want to bribe the locals to get it done efficiently. If you find the right local person get involved, they will help you out.
The most frustrating thing of doing something in SL is the corruption. Which is the fabric that joins everything. Those who can navigate that well suceed.
Good luck!
