With the viewfinder-less pinhole cameras there's only one thing that works: getting to know the camera and how it sees by making a whole stack of photos. With a new box, I tend to spend a lot of time crouching behind the camera, holding my arms out, trying to work out the edges of the field of view (using the dark slides helps, too, along the angles to frame what should be on the image.) You could try using some paper negatives for the first few batches, so you don't feel like you're bleeding money while getting to know the new camera.
Another thought: maybe rig up a piece of ground glass and a viewing pinhole for composing images on the 8x10, at least until the camera is more familiar? You could probably make a holder for the glass (out of foamboard or something) that would fit in the back like a DDS.
Contact printing full negatives is one hell of a disciplined approach. But so hugely satisfying when it's right.
(As to limited holders: a dark bag and a spare box of film are good things to carry along.)