Funny thing that you would create such a mod because (I don't know if you now) the Germans actually experimented with Type XXI conning tower on type IX. I found just a snippet of information about it.
Here is what I posted on AMP website a while ago:
Type XXI conning tower on type IXA (U-37)
I have recently came across a drawing in Waldemar Trojca's U-bootwaffe 1939-1945 cz.4 (page 38) which shows a type XXI conning tower on type IXA.
The Polish captioning next to the drawing says: Doswiadczalna nadbudowka zamontowana na U 37 (typ "IX A").
I think it means "Experimental superstructure mounted on U 37"
Did anybody here heard about such a thing? Did Germans really tried to cross-platform project like this?
I have tried multiple Google searches but came up empty.
I just think that it might be an interesting idea for a conversion.
Here is the reference picture:
