As an adult I have tried wearing a size 8 diapers and I have a 32 inch waist size and a slim build.
While the size 8 diapers do not fit properly on me as I would expect, I can make them fit somewhat by first taking off the Velcro strip and also I peel the cloth backing off when I can.
Then I use a glue gun and after tearing the Velcro landing strip of the diaper in half, I use the hot glue gun and glue one piece of the landing strip to each side of the diaper.
Then the tabs are able to stretch good enough to reach the Velcro strip.
But I also need to and always do run some packing tape around the diaper waistband and over the tabs to keep the diaper from coming off and sliding down.
The size 8 diapers are designed for older kids that are 46 lbs and up and won't fit an average and healthy sized adult perfectly but you can make them fit and work for fun.
I mainly wear the size 8 diapers and even size 7 diapers for fun and not for function.
I just enjoy wearing an actual toddler and kid diaper once in awhile.
Adult diapers feel much better, but are not as fun, unless you're wearing the more expensive adult baby diapers.
I can even make a size 6 Pampers Baby dry diaper fit somewhat pretty good by also removing the cloth backing and Velcro strips and gluing the Velcro landing strip pieces to the sides of the diaper and also running packing tape around it.