Lets see,..... 25" fuselage length X.75= slightly over 18" length. This is not an exact science, but 18" is a lot longer than 11.5".
I also have an old set of those floats you have. They were made by Cox for a 1/2A control line cub on floats and I too have been eyeing them for an application. Right now they are going to be the tip floats on a 2X25 seaplane I am designing. I think they will be about right for that application; especially since length does not matter here.
If the Kavan floats are too pricy as mentioned by mpowrmnt a foam, flat bottom square cross section with the step under the CG will work. Use the profile of the floats you have as an approximate guide for the profile cross-section. Glue strip of spruce in a groove along the top edge to screw the attachments to. Floats should be RIGIDLY mounted. This means no flex. No finish is required but you could paint or cover them with an iron on covering. Epoxy will weigh too much.
Another rule of thumb is that one float should have enough bouyancy to support the entire weight of the aircraft. Look at photos posted elsewhere to see what others are doing.
I think I'm running out of thumbs.
Keep us posted.
Peter