Frame 3? - Cutting? It is supplied as one-piece port-to-starboard, probably to ensure correct hull shape. I guess you'd be hard pressed to fit the case without cutting it.
IIRC we took a little off at a time till it was right.
Graham's comment "take it easy" is especially significant when shaping the underside of the "logs" to fit the actual line of the sole. Take off a little at a time, and keep checking, eg with paper/card as a feeler gauge. Don't stand in the boat when doing this, because distortion of the sole gives a false reading.
Screwing - YES - Screw it.
Working alone? Can you get someone to slip bricks under your cradle while you lift one end at a time?
Clamping - Peter's threaded rods are a great idea. Graham seems to have used long clamps. IIRC we used car batteries. And I think we used odd lengths of wood, jammed between the case and the buoyancy/lockers, to ensure the case remained vertical.
FWIW - We subsequently in-filled the space, between the logs, each end of the case, to prevent water and debris loitering.
See Here. Not annotated, but evident.
Further pics available via the "Proteus" link in the table of Known UK Morbics.
PW