Depending upon how detailed you want to go, you could kit bash the old Revell 1/125 U47 or U99. See comment below.

I have seen two diaoramas in Fine Scale Modeler (our American readers will no doubt be familiar with this magazine, & very good it is too). One depicted a section of a Type VIIc (or Type IX, memory fading) centring on the conning tower. The crew are at AA stations and are depicted shooting down a Catalina.

The sub is approx 1/35 (big) with crew and guns to match (rebuilt Tamiya?) the Catalina is 1/144 to give an impression of distance with a small motor in one engine to make the prop turn. The other has been knocked out by gunfire.

Another set piece is a Grumman Avenger to 1/72 posed banking over a 1/125 VIIc converted and truncated from U99 (cut down to waterline and just aft of the tower, to fit the base board). The differing scales and the enforced perspective make for an interesting model.