Here are some design concepts for smaller 1atm. submarines: 

"2or3at3,5t" is a 3 seater which can be also be use as double seater
with a a diver lockout hatch at the bottom - Weight about 3,5 t 
I think this is the smallest size you can have a Lockouthatch. 

"Fast2man" is a also a double shell boat for which the abbility to move
fast and manoverable. Was designed for rivers with currents -
Were you have to run 5 knots just to go 2 over ground. 

"Littleone0,5t" is a very small 0,5 t single seater - useful only in
current free water for periods of 1-2 hours. Can be travel even
behind Italy cars.. 

"Minesweeper2man" was a concept to use for the outside the in large
quantity available "Minesweepe" hulls. This concept with just 
about 25 % greater dimensions has a great potential..

"Onemandrysub" shows just the generalconcept for a standard homebuild
- simple and cheap to build boat. 

"Overview01" shows the range possible for selfbuilders. 

"Twomandrysub" is very simillar to Onemandrysub - just for two. 

>From all concept a DXF release 12 CAD file exist - 
its for free - if you have a serious interesst - just order it at
carsten@euronaut.org - but be carefull the files are 2-3 MB big 
and the concept comes without any guarantee for nothing. 

regards Carsten