The first step is determining the interior dimension of your radiator cover, allowing clearance for heat all the way around the unit (Image 1). It may be useful to make a simple drawing to assist in the cutting and assembly phases of the project (Image 2).
With your dimensions and drawing in hand you are ready to cut the top and the two side panels. Use a table saw to rip down the lumber to your desired width. Don't concern yourself with cutting the lumber to the proper height (or length) yet. That will be done in the next step.
You will need to use a strong wood material that can withstand the high heat output from the radiator, such as 4' x 8' sheets of 3/4" plywood with a birch veneer. The birch veneer will allow you to easily sand and paint (or stain) the unit.
Next, take your top and side panels to a miter saw and cut them down to the appropriate length. The miter saw will make a cleaner, more controlled cut than the table saw, and is a better choice for smaller pieces of lumber. If your dimensions are too large to be cut using your miter saw, the cuts can be made on the table saw, but take great care to make the cuts as clean as possible.
If you’re building a radiator cover for the kitchen you may want to give it a custom look. Consider contacting your cabinet manufacturer to purchase materials similar to your kitchen cabinetry.
With the exterior panels cut for the unit, it's time to cut the face framing. Use 1-1/4" poplar, cut to the same dimensions as the exterior panels using the miter saw. The angles should be joined with 45 degree angle cuts for a smooth, tailored look. The point of this face framing is to hide the unfinished plywood edges.
Now that all the pieces are ripped down and cut to size you’re ready for assembly. You will start with the two side panels, then the top, and then the back support strip. Assemble the pieces using a nail gun and construction adhesive.
Do not forget to install the back support strip. This will keep the unit square and from wobbling. This is especially important if you're installing a custom countertop to the unit.
With the frame fully assembled, it's time to glue and nail the face framing onto the front of the unit. Line the outside edge of the poplar face framing up with the outside edge of the plywood frame so that everything is flush. The face framing should extend inward, giving approximately 1/2" of overlap on the inside edge of the unit. It's essential that this overlap is entirely contained inside the framework, as the back side of the face framing will be used as the mounting location for the decorative aluminum sheeting.
Next, install the decorative aluminum sheet. Trim it to the desired size with a pair of tin snips or heavy-duty sheers (Image 1). Once cut, place it on the interior of the face frame and use a staple gun to secure it (Image 2).
Before installing the radiator cover, consider installing a heat shield to diffuse the heat between the top of the radiator and the top of the cover. This step isn't entirely necessary, but if you plan to install a countertop or simple use the top as a shelf, this will limit the transfer of heat to that surface.
To install the radiator cover, simply slip it over top of your existing radiator. If built correctly, the unit should be sturdy and capable of freely standing. It should not need to be affixed to the wall or the floor in any way.
If you would like to add more design interest or functionality to your radiator cover, consider having a professional create a template for a countertop.