I live alone, so have to limit anything with restraints or bondage to stuff I can do myself, but have discovered a really good setup that gives that 'locked in' feeling but you can do it all youself.
Fist I put on a snappy bodysuit. I have one where the snaps are at the shoulders instead of the crotch that makes the get up work much better. With the crotch ones you can get changed without having to take the onsie off, though difficult you can do it just by taking the top half off. The shoulder snaps however, mean you have to take everything off. After this I put on a adult onsie I got a while back. It has feet attached and the cuffs are those thumbhole ones, which help with the next bit. Finally I have a collar and some cuffs, both for my wrists and ankles. They're just buckles but once on, make me unable to remove the onsie without taking them off. The collar blocks access to the zip once done up, the thumbhole cuffs means I can't just pull them under wrist cuffs, and the attached feet mean I can't slip my feet through either.
It's amazing. Sure, you can get undressed yourself but the combination of everything just adds that extra level of effort. As mentioned, the combination means if you want to change your diaper, you have to take everything off too. If you really do want to lock yourself in you could easily padlock the collar and cuffs and put the keys in a bowl of water and freeze them, now you have no chioce, you have to sit in whatever mess you make! Course, if you have a partner you can give them the keys too.