PA23 pilot reported the main cabin door opened in flight; causing a pitch down and loss of altitude until the door could be held closed. Pilot landed safely.

Narrative

While in cruise phase of flight doing aerial survey; the main cabin door became unlatched and popped open. Initial response from aircraft was an uncommanded nose down pitch of 20-30 degrees. Immediate recovery to straight and level flight initiated and regained control of aircraft with only 100-200 feet of altitude loss. Door would not close properly so I had to hold it closed while also flying the aircraft. Control of aircraft was maintained and all checklists followed to come back and land safely at nearest airport. No injury to persons or damage to aircraft/property occurred.Probable cause of door opening was most likely forgetting to securely latch the locking bar before takeoff. The door handle slowly loosened and eventually opened. Hot day/aircraft and wanting to get the engines started and moving was most likely cause of missing of locking the door. Moving forward; taking precautions to ensure door is securely closed and locked once all persons are inside aircraft is what can be done to prevent further issues.