Made one myself from a camera stand topped by the head of a rubber plunger. Easy to do - with some patience it gives you a good, but not inflexible playing position with the alto. It is not ideal, though, as you have to choose the right chair and work on your sitting position to get the plunger as close to your crotch as possible. It can be used in a standing position too, if you have room for it, but it might get awkward on stage ;-) Still, it helped me out. Another option is the Sax-on from quodlibet.com. I have used their Phred for clarinet, and that was a good piece of equipment.
Best of luck
Jesper