Why can’t you burrito them? I wasn’t sure if you meant you haven’t had success doing it, or if you can’t even do it that way for some reason. If it’s the former, you aren’t doing it right (which I know is easier said that done with a struggling bun!)
What position are you holding them in? For me, the easiest way (for both cats and buns) is sitting them on my lap kind of like a baby. I can use my thighs, chin/head, left hand, left arm and right arm to restrain and still be able to clip with my right hand.
All I can say for the meantime is keep at it. Go slow (1-2 nails at a time) so it’s not a horrible ordeal for them, and give them treats before/during/after. They will mellow out and get used to it. I have had luck covering their eyes.
You can also try local groomers, some may have experience with rabbits and your rabbits may be “stress-tame” during the visit. But IMO slow and steady is best. You will get there!