Try cheap and easy first... put the diaper on backwards, put the bottoms on backwards (if they are not elastic waist), put baby in a sleeper on backwards. Do any combination of the above at the same time.
DS1 does often take off his diaper during nap, but putting him to bed with his bottoms on usually prevents him from doing so. It's not that he is unable to take off his bottoms himself and then get to the diaper, he just doesn't usually bother if he has that extra layer on.
Advice and commentary on living overseas
DD1 15, DD2 12, and DS 9