To modify baby diapers to fit adults take the landing strip off the front of the diaper which is Velcro and then cut the landing strip in half.
Then glue the landing strip halves at each end of the diaper waistband and extend it out a little.
Use a glue gun to glue the landing strip to the sides of the diaper and then you can usually stretch the diaper tabs to fit to the edges of the diaper.
I did this today to a Luvs size 6 diaper and it fits me some and the tabs are not popping off.
You can also attach tabs from another diaper to the baby diaper you're wearing which will extend the diaper tabs enough to make them fit.
To fit into baby diapers you need to be pretty skinny and get the largest size available such as size 6 or size 7.
Gentle Steps size 7 diapers can easily fit my 32 inch waist and they swell up quite a bit.
I soaked a gentle steps size 7 diaper today and just changed into a Luvs size 6 diaper.