The fee has to be reasonable, and if the QDRO is complex or if the parties engage in extended negotiations that yield multiple draft DROs for review, then hourly fees are appropriate. Having a fixed fee makes sense for a "standard" QDRO where there is one DRO presented that is drafted properly and divides the assets between the participant and one alternate payee using reasonable time frames. If the practitioner wishes to avoid using hourly fees, consider having a fixed fee for the various parts of the process - a fee for each DRO reviewed, a fee for each additional alternate payee, and a fee for each year of data history needed.
If there was a way to do it, I would add an incompetence fee. I am amazed at the number of DROs have the wrong plan name because the drafter used a DRO for a different client as a model and did not proof read the DRO before submitting it for review.