Andrew,
I made one some years ago for a large TOS lathe and I needed it in a hurry.
I finished up 'designing' a two part hinged opening type.

Sorry a bit blurred but it's about 24" on the tape.
The lower half consists of 4 pieces of 10mm thick plate laser cut all the same and the Vee's have been cut in to fit the bed.
The top half is again 4 pieces all the same but just semi circular.
The bottom and top are 'paired' by cutting off two of the ears on each end and the opposites on the other half.

The whole lot if fastened together by countersunk allen headed screws. Once each half has been bolted up two 17.5mm holes were drill in each and then tapped 20 x 2.5mm to take 4 pieces of studding.
Once the laser cutters had delivered the 8 plates it took about 1/2 a day to get to completed.

Steady in action.
John S.