ORIGINAL: Fluid
In the US this is available from the LHS, and is usually 2 oz/sq yd woven fiberglass cloth. He's right, 9mm is way too heavy. 3mm would be plenty for a .25LA, and even 1.5mm is not too light if you used 3mm frames. What are the dimensions of the boat? Airboats with that power level are usually in the 600-700 mm range.
Bottom is 8" wide so about 232mm. Top is double that so about 464mm. Length of the bottom is 24" so about 696mm, length of top is 29" so about 823mm.
What do you mean by frames?
Thanks,
Marc.