I've never purchased any so can't comment on the commercial ones. I just used what I found lying around.
Get two different sizes of empty/clean long neck wine bottles -- the regular size 750 ml and the double size 1.5 Liter. Place them on a flat surface and practice stroking into each size (it develops a level stroke and forces you to change your bridge height). Then, if you want to adjust the angle, put something at the rear of the bottle to hold it in place and something else under the front to jack it up a bit.
Start out stroking slowly and gradually increase the frequency to as fast as you can go without touching the inside of the neck of the bottle.
I've never had any problem not being able to avoid touching the inside of the the bottle's neck so I never used them much. I only used them to confirm that I didn't feel the need to spend bucks on a plastic commercial one like I'd seen on-line.
Thanks for starting this thread. I'm interested to hear what others have to say about the different commercial products.