Product review (1 stars): Grossly overpriced at best, inefficient at worst...
Most standard adjustable air deflectors are fairly inexpensive ($<5 ea), constructed of a rigid plastic, and use magnets to attach to the register. Given the $13 cost of this "extended" version, I expected the product to be of similar or better quality.

Alas, I regret not reading the negative reviews. As others have noted, this deflector reminds me of the cheap disposable liners used for paint trays. It is made of a very thin plastic, and only a small plastic clip is provided to hold it to the register.

I needed this device to extend out almost the entire length. Unfortunately, because of its flimsy design, it simply wouldn't stay in place.  The cost of returning it forced me to be a little creative. I put a magnetic adjustable deflector over top of the EXTVENT to hold it in place, and then I used the provided clip to hold the extensions together.