The depth at the kick panel area is about 3" minimum near the door jamb, increasing as you get closer to the firewall. On my early 250SL, there is also a vertical support at about 3- 3/4" forward of the door jamb and runs from the fender to the kick panel steel. It starts at the floor and goes up to the dash.
I used 4" Isophon speakers installed in original vintage square Becker speaker enclosures. The speakers mount to the inner metal frame of the enclosures with tubular rivets in the corners, exactly as the original. The speakers mount in the area between the door jamb and the vertical support. Center of hole is at 81mm from the bottom trim and 61mm from the jamb trim per Becker specifications. The old enclosures place the speakers about 7/8" into the foot well, so the hole is only for the speaker magnet. I hope this helps.
Tom
1967 250SL
Los Angeles