Here are some of the devices that I use to secure wood while carving. First up is the end-mounted bench vise. A bench vise like this one is very useful for either directly clamping your work or for holding other clamping devices. Note the wooden jaw plates attached to this vise to prevent the metal jaws from marring the work.
Another device that can be used to attach carving to the knee is a piece of plywood with a carriage run through a counter sunk hole. the platform is then screwed to the carving.
This device is also clamped onto the knee using a threaded handle. This works particularly well with smaller carvings since you can use smaller screws and still get a good hold on the carving. Both of these devices used in conjunction with a carving knee give a tremendous range of angles to work with and are easy to adjust.
For relief carvings, I especially like an adjustable stand like the one above. It proves a good work surface with two handles to clamp the work and will tilt to any angle from horizontal to vertical. I came by this one around 20 years ago. I have not been able to find another like it. If you know of a good source for adjustable stands like this, please let me know.