1. Rigid Box Construction
Apart from die cast boxes the usual way to provide small or medium size rigid enclosures is to use pieces of pre-cut metal sheet held together with 1/2inch (12mm) angle sections. This works fine but the angle takes up a lot of space and requires fixing nuts (or hank-bushes) on the inside surfaces which can be difficult to get at in a small assembly.
An alternative solution is to use four pieces of 1/4inch (6mm) square bar instead of the angle to hold the side sheets together. The bar is drilled and tapped 6BA (or M2.5 - M3) in the sides and ends to provide fixings for all sides of the box. This takes up less space inside the box and removes the need for fixing nuts.
The choice of metal is down to the preferences of the individual constructor but aluminium is usually the easiest for home working and does not require surface finishing in the same way that steel does. 'NS4 - half hard' 18swg aluminium sheet is an ideal type of material for the sheet sides as it is rigid and drills well but is too hard to be bent - cut it with a saw rather than tin-snips. Make sure that your tap is correctly lubricated and use the correct size drill for the hole to be threaded. Clean all of the metal pieces after the machining is completed.
If you want to finish the metal pieces, aluminium should be Alocromed which provides a protective coating which is electrically conductive (Anodising gives an electrically insulated coating).
I have used this technique to construct a box for a 5MHz VFO in a mobile/holiday rig with considerable success.
2. Box Cleaning
I have collected a number of Heathkit units over the years and most of the cases have gathered a lot of marks, cigarette smoke and whatever else lurked in previous owners shacks. I have found the best way to clean the cases (without the associated electronics!!!) is to give them a warm bath.
Remove the internal chassis and front panel assembly, mounting feet etc and run some warm water into the bath. If the case has sharp corners then put a rubber mat or similar in the bath to protect the bath surfaces. A non abrasive kitchen cleaner will remove the accumulated dust and dirt - use a soft nail brush to remove stubborn marks and a used tooth brush to get into the corners. Follow with a good rinse, drain, promptly dry with a soft towel and leave for a few hours in a warm airing cupboard to complete the drying process. A suitable lubricating oil should be sparingly applied to any moving parts (hinges, threaded holes etc) to prevent corrosion and finish the job. Front panel knobs will also benefit from this cleaning process.
Caution must be exercised because the paint on very old units may be brittle or chipped and may come off very easily - treat it with care and it will look great and last a long time!