If you are at least able to bend your arm at the elbow then you can do so and rest the rifle on top of your right inner elbow while your right hand/cast is placed close to your shoulder...this will place the rifle's centre-weight on the elbow making it easier to control the shot.
Wrapping the sling(if you have one) around your right arm by placing it through the sling and then back out towards the right and then up and over the sling to wrap it against the arm, and then bringing your cast then back towards your shoulder and thusly under the rifle butt stock is how you do it. This will help steady the rifle and marry it to your body(upper torso). Tighten or loosen the sling accordingly until you get a firm tight lock with your support arm.
If you have no sling then you could always try and steady the rifle in a similar manner using your inner elbow but while sitting on the ground with one knee raised and resting your elbow thusly on your knee, rifle cradled again in your inner elbow joint
There is no reason why this shouldn't be possible, unless of course your entire right arm is in a cast