My first thoughts were to use the same type of rails and drives as my Concorde printer viz. V-Slot aluminium extrusion with Mini V wheels including C-Beam extrusion and gantry plate carriages for the Z axis.  I am now looking into higher precision rail systems on a smaller scale.  Home made linear bearings using tiny ball bearings arranged in threes around round rods is one option but this means using dozens of ball bearings with every third being adjustable.  Although giving high precision, this is very labour intensive and time consuming, even with 3D printed parts.  My thanks to @Chriske for this idea.  Another possibility, apart from the ubiquitous cylindrical linear bearings on round rods, is these MGN12 rails.  A combination approach is also a possibility.  I shall be giving this a lot more thought before going ahead and ordering anything.

The piece of hardwood ply I bought for the job is 900mm x 900mm x 18mm.  To allow room for motors, brackets etc. I reckon to add 50mm at each end of the XY rails.  So that makes 400mm x 400mm inside and about 440mm x 440mm outside.

This is the sort of box I'm thinking of with a height of just under 450mm.  This should give a print volume of 190mm x 190mm x 220mm approximately using 6 off 300mm MGN12 rails.  The bottom front rail will come from the bottom panel and the top a piece of ply left over from my Concorde printer box.

Assembly top view showing box, X and Y rails plus crossed rods that carry the extruder and hotend etc.

Sliders for X and Y rails added.  So far this is just a small number of the many parts involved in this 3D printer but I think a CAD model is worth drawing to fit all the parts together.

I'm going to try a smaller box and see how that goes.  This is using the length of round rods I have on order which can be attached to the sliders with 3D printed brackets.

Drive belts and bed.

Filament spool in bottom of box.  It will be supported on a centre spindle running on ball bearings and central axle.

Cutting pattern for the plywood.

Assembly update.

Started printed parts production starting with the bracket that attaches to the mini rail slider and takes the cross-bars for X and Y.  The holes on the top are to take bolts and straps to hold the ends of the timing belt.  Design was altered to make 3D printing easier.

Front.

Back

Assembly with box hidden.

If the belts run with centre line 25mm from the sides of the box, the pulleys and hence want to come to 20mm from the box sides.  So with X=440 and Y=340 the axles will be 400mm and 300mm.

Update...

The X and Y drive systems.

Went on to design the XY Carriage - what on a standard 3D printer would be the X Carriage but in this case runs on two round bars at right-angles.  This carries extruder & motor, hotend with heater block and nozzle plus Z probe and cooling fans for the hotend heatsink and parts-cooler.  Four linear bearings run on the two cross-bars.  As a result of the design model and comparison with the print bed size, the depth has had to be increased a bit to give full bed cover.

Here are some screenshots.

Now to work out what depth the printer needs to accommodate the extra Y dimension.  The full Y range can be produced with a 362mm Y bar and 2x 13mm thickness of mini linear rail.  This gives a total of 362 + 26 = 388mm.  To allow for tolerances I think I'll make this 390mm.

Next to work out how this can be cut from the 900mm x 900mm x 18mm sheet of plywood.  The inside width has already been determined as 410mm.  Since the back, front and base fit inside the sides, this will be the cut width.  Overall height of the box (outside) will be half the height of the ply less cutting, so a little less than 450mm.  Width of the side panels will be 390mm (from above) plus twice the plywood thickness ie. 390 + 2x18 = 390 + 36 = 426mm.  Base will be 410mm x 390mm.  This gives a total width of plywood of 410 + 426 + 5(ish) = 841mm Add another 5mm for the cutting gives 846mm and therefore just 54mm wide strip left.

Adding up the back and base from the height of the board, we get 457.5mm + 5mm + 390mm + 5mm = 857.5mm leaving a strip of about 42.5mm.  I was hoping it would be 100mm to use as the top front piece.  However, I have some 18mm plywood left over from making my Concorde printer that I can use in necessary.

The hotend heatsink cooling on this printer uses a radial fan without any ducting to concentrate the airflow through the fins.  To test that this would be adequate I made a small modification to the same part on my Concorde printer then checked the heatsink temperature when printing PETG at 240°C - it ran quite cool even with only half the heatsink being cooled.

Latest assembly.  XY Carriage with running rods added and also drive shafts with timing pulleys.

This screenshot shows the latest XY Carriage with the partscooling fan and air duct moved underneath so that it no longer sticks out beyond the frame.

Plywood Cutting.

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