Hi Jack, I drew a cylinder and 2 rectangles like this:



Select the 2 rectangles and use Construct > Boolean > Union on them:



Select the cylinder, and run Edit > Trim. Pick the rectangles as the cutting object (Trim has projection of planar curves used as cutting objects built in so you do not need to do a separate projection step). At the prompt in the Trim command where you select what pieces to discard just right-click or push "Done" without selecting anything to keep all the pieces.

Select this piece and run Construct > Offset > Shell to thicken it into a solid.



That will make this piece:



Select the outer face of that piece and delete it:



Join the pieces together into a solid:



- Michael