A couple of RNLI offerings.....

[scooter]

I still have the Airfix Severn in my stash.  Need to source a decent crew and then decide if I want to depict a rescue or just underway.

See if you can find one of those fancy yacht kits, and do a diorama of a sinking yacht rescue.

[Nick]

If it was an Arun-class lifeboat you could have it towing an S-61 - https://www.pprune.org/rotorheads/609542-s61-g-beid-tow-into-aberdeen-july-1980-a.html

Or do a modern what-if model with the Severn boat towing an Mv-22 Osprey 

[PR19_Kit]

How about an Atlantic 21 towing a Robinson R-22 on floats? 

[Rheged]

Given that the newly refitted, reengined Severns have 3200 horsepower (two MTU 10V2000 M94 engines at 1,600 hp each)  to play with, I suggest that the diorama can accurately show them towing just about any flying machine.