This is a set of templates for SketchUp. It's made in v4 (demo), can be edited in v5 (freeware).
The box has a baseplate made from 12 mm thick 500 mm x 300 mm standard stock sold by RS Components.
The other panels are from similar stock, or lesser grade aluminium, 3mm thick.
The angle extrusion is 3/4" (19.05 mm), with thickness of 1/8" (3.175 mm).

The fixing hardware is stainless steel, M4, hex head countersunk machine screws with allen key drive,
two lengths, 10 mm and 20 mm. Nuts are either M4 full nuts or swage/clinch nuts chosen for 3 mm sheet.

Parts are formed by lines and planes, placed to form the modelled part and grouped so they can be moved
and copied as a single item. Some groups are also components. Editing one instance of a defined component
will edit ALL instances of a component. This makes it easier to edit fixing hardware, and angle sections.
Panels are not components, because each one will likely need different edits.

The panel cutout templates are easy to use. Open the group of lines for the cutout you want and copy them.
(The copy remains on the Windows Clipboard so you can close SketchUp between copy/paste operations.)
Open the panel part to edit, entering through nested groups until you are at line/plane level for editing.
Paste the cutout on the surface, clear of any existing line, then use the Extrusion tool to punch through
to the other side.

The box model contains a few 'materials' for colour coding parts for the materials they're modelling. One
is an actual texture, for closed cell polyethylene, a cheap and useful thermal insulator.

MAJOR hint: SketchUp is entirely 'intuitive' in operation. If YOU can't see the inference points you need
when moving, copying, cutting, drawing, whatever... the program can't see them either. Most confusion will
be cured if you remember this. Hide, Zoom, Orbit, Pan tools will change view for clarity. (Orbit combines
ALL moves you normally need). Hide also removes things from the view so that SketchUp will work faster
with what is still visible. Use temporary construction geometry. Placing this outside the group before you
go in to edit it is often better than placing it inside with the edits you're making.

These models are made on a metre scale. 1 METRE is a real-world millimetre. That's a useful habit to get 6
digit accuracy without errors in Sketchup. It used to be needed in v2, it probably isn't now. Scale down
1000 in finished result if you want to export to DXF for other CAD programs. Or let them scale if they're
better at it.

SketchUp isn't designed for building cars or laser scanner motors, but if you want to make models of 3D
systems of lines and planes, with enough realism to stop you making expensive mistakes with hardware, it's
more than good enough, and it's free for personal use. If you can find a v4 you can run it on W98. v5 needs
WXP. There isn't one for Vista yet but that won't concern me. I won't stay with a Microsoft OS beyond WXP.