If I had a big fleet of adult trikes, I'd definitely make two truing stands. One with round rod, the other with a d shape. Easy way to make this would be to take a beat up old trike rear end and build from that. But you could just grind down a rod to d shape. Then literally retain the wheel on that rod with a vise grip grabbing the rod. The d shaped rod would have to rotate on a bearing or at least a bushing though, which is why the easy way would be to just buy a trike rear end if you did lots of this job.
A good truing stand is a joy, that's for sure. But a motor wheel on a bike, I just do the zip tie trick unless I happen to have the motor off the bike.
When I had three of these trikes going on the job, I had plenty of bent wheels. The condo complex had some speed bumps, and with 100 pounds in the basket the back wheels took a pounding. I re laced several wheels to better rims, and constantly had to tune the not so ruined wheels. Like I said, a very crude truing device, a pile of bricks, was plenty to let me eyball the dang thing to within a quarter inch of straight. You could run them worse than that, provided the spokes were all tensioned snug, but I did like to get them straight as possible, about once a month. You can also true to something attached to the basket, if the basket is rigid enough. Mine were wobbly folding baskets, so I did not do that.