Just make one. I hemmed and hawed for months, then bit the bullet. While it is not the best I have ever used, it allows me a ton of freedom. I've worked in the garage, out in the yard, up at the gun club, anywhere I can anchor the bottom.
One change I am going to make is to put belt leather straps on the ropes to hold the stave. When the bow gets smaller and smoother, it seems to slip just a bit. But, there's usually not that much force at that stage, so it isn't a huge problem at all.
Overall I am very happy with it, and the cost was pretty minimal.
Kyle