Let me get this right, you have a radio set and some form of air propulsion to power an air boat.
I guess that it is an electric motor of some sort ??

To help you here, you are going to have to give us a whole lot more information.
What do you have now ?
What do you want ?
Do you know what sort of hull you would like ?
Can you make / fabricate components ?
What is your RC experience?

I know this looks like some sort of interview and I make no apologies for that. The reason is that there are many different types of hull before we even get into sizes of them.
I have a number of different hull types and have worked from the bottom upwards on a budget.

I am very keen to offer advice and suggestions as are many of the other members here.
Give us more to work with.