Forbech The trick is to use the lightweight foot together with a plywood hand rest. Then the weight of your hand will hold the base of the space mouse stable as you manipulate the control knob with your fingers.
More info here: http://www.4tech.dk/foot.htm
I use a laser cut plate as hand rest but other people have designed and 3D printed hand rest:
A hand rest will also help to keep the space mouse in the right “direction”. The round foot tends to turn during use and when the space mouse get misaligned the navigation gets very confusing