I have recently bought the same floats and noticed the same thing.

With respect to the holes in the top, I assumed they were to let air in and out so I left them as is. With the float attached, the seal seemed good enough that water did not enter the float. I've had a few flights with no leaks. I was more concerened about the seal around the pushrod for the rudder (I used a servo in the float), but the water has not got in there either.

I just used the wheel collars as supplied. One thing is that the floats can move to from side to side so you definately need something to keep them from sliding off.

The only issue I had was with the float mounts - I have a flat bottomed plane and had to use blocks to get the floats in the right place. Hopefully they will fit your cub without much modification.

Also, I needed to add a lot of weight on the left float to get the plane to balance properly - doesn't look that good but the servo controlled rudder is a lot better than the cable system I've used on other float planes.