Interesting thread on SB, where a guy made a float using some sort of sleeve made of a reflective fabric, and inserted two 2 liter soda bottles. Here's the original website: http://apox.ru/content/view/931/1/
Somebody pointed out that the bottles might crush at depth, and someone else pointed out the interesting possibility of adding a little vinegar and baking soda inside each bottle to generate a bit of positive internal pressure before capping off. I must admit, if you could get the amounts right, this seems like one of those "elegantly simple" ways to do it, vs. any kind of inflator valve.
It seemed like an interesting idea. You could construct a sleeve of heavy duty material, as long as you wanted, and add as many bottles as you wanted. About 4-lbs buoyancy each. Potentially, at least, it could be a dirt cheap float. If the fabric sleeve were durable, just replace the soda bottles now and then if they leak. If the float were comprised of 4-6 bottles, there would even be a fair bit of redundancy if one bottle leaked.
So what would be the disadvantages of a long, narrow float? Seems like it would tow easily through the water. Should exert very little drag on the fish if he made a fast run, and would rely only on buoyancy.
What is the downside?