I would recommend Treatments that Work's Unified Protocol for the Trans-diagnostic Treatment of Internalizing Disorders. There are paired clinician and patient books with a chapter on interroceptive exposures, and the clinician book has excellent instructions on guiding patients through an interroceptive exposure.
As far as patients' experiences in the exposure not feeling realistic, I would ask about what sensation is missing, since different exposure activities will be linked to different panic sensations (e.g., jumping jacks to raise heart rate, spinning in a rolling chair to induce dizziness, breathing through a small straw to induce light headedness). I find it helpful to go through the activities yourself to see what they will feel like for clients. If the symptoms do match up with a patient's experience of panic but aren't intense enough to feel realistic, there are many ways to safely heighten the exposure such as increasing the duration by 1-2 minutes, spinning in a chair with your eyes closed, or breathing through a small coffee straw instead of a regular straw. You could also chain or combine some exposure activities (keeping a mind toward safety--you don't want to induce dizziness and then get on a treadmill for example) such as jogging in a surgical mask or breathing through a straw while spinning on a chair. As with most aspects of cognitive therapy, it can be an iterative process of arriving at the best exposure activities for a particular client through a curious approach and trial and error.