That would work too, I would just cut a HEPA bag and glue that to the filter frame. You'll get better airflow and more filtration, as these don't have the best filtration with those old 5033 bags. Although in their time they were probably about as good as it got, since it was considered "triple" filtration - bag, foam premotor filter, and foam exhaust filter.
Sears did make 5033/E allergen micro bags for these more recently, but they seem very uncommon - I've only ever seen one pack of them on eBay and I never saw any in Sears.