I'm sure you've seen many "professionally" made male chastity devices on the internet. The common theme is a method to restrain the "occupant" from establishing an erection without preventing normal bodily functionality.
The most common (available at most any store if not owned already) male "restraining" clothing is an athletic supporter (jock strap). It's primary purpose is protection of the male testicles but it also restrains the penis somewhat.
Starting with a jock strap, modifications could be added (some sewing might be necessary) like a chain or leather strap to which a locking device could be connected to ensure "controlled" entrapment. Furthermore, wearing a protective cup (common for baseball, hockey and football players) would prevent erection. A slight modification with a "discharge" tube would allow disposal of urine when needed. Of course, if bladder control is also on the "training" schedule, this may be omitted.