i recently had to keep a little duckling in a towel donut as well..its hard! i had the towel rolled up, with a paper towel rolled up inside it, so that he was kind of propped up to regain strength in his legs. i think its actually a good sign hes getting out of the donut, means his foots healing.
my duck had a problem with his tendons and couldnt stand or walk. vet recommended hobbles, and after a day or two he could stand, but he wasnt standing on his feet right. the webs were closed.so i actually got the moleskin pads people use for their feet cut it in triangles and put them on the bottom of his feet to keep them open. after two more days, his webs stayed open and he was taking little steps. soon after he was walking.
its stressful because you feel so bad for the duckling..but if its will to live is strong, itll make it, and not even look like it was an injured duck. mine walks around like it could always use its legs, hes 4 weeks old now.
good luck!