Black Foamcore Sliding Box
If budget were your main concern, and wanted to tackle the problem with a DIY approach, you may want to look to camera designs from the formative years of photography, specifically look at the
George Eastman online museum. Here you will find lots of good ideas you could borrow, among the easiest being the nested, sliding box design.
I built such a camera from black foamcore, including my own version of a sheet film holder, similar to that illustrated in the book "Primitive Photography". I used a removable view screen frame, built to the same thickness as the sheet film holder, with a sheet of translucent plastic at the same location as the film plane would be in the holder.
I use an objective lens cell from a pair of 7x50mm binoculars for the camera lens; my lens will cover 5"x8" format fine with no movements. I use the lens wide open (at F3) for focus and composition, then stop down to around F50 with a simple black disc, with central aperture, placed over the front lens.
For the shutter, I happen to be using paper negatives or APHS ortho lith film, which has a 'speed' of around 5, so a simple lens cap shutter, timed manually, works fine. For faster film, a mechanical shutter would have to be used.
For your application with macro photography, just ensure that the nesting boxes will telescope out to the proper focal length with adequate overlap to ensure good mechanical stability. Of course, most nested box designs from the 19th century use a baseplate, with the 'fixed' half of the box permanently attached to the plate, and the moveable half of the box sliding in and out on grooves; such a design can be very stable.
Good luck and keep us posted on your progress.
Edit: For a more permanent material than foamcore board, you may want to consider multi-laminate model aircraft plywood. Using two layers of such plywood, with the sizes of both layers staggered at the edges of the panels, you can get a rabbited joint that's a very good assembly method in terms of both strength and light-tightness, without the need for fancy router tools.
Also, I'm assuming for macro-photography, you may be wanting a rather wide aperture so as to isolate your subject with a somewhat narrow DOF; in which case the resulting shutter speed will definitely necessitate the use of a mechanical shutter, even for slow ortho media such as what I use.
~Joe