For lack of any big tools like belt sanders, etc., I use a little $45 orbital palm sander I have on hand for little home repair jobs. I just cut a piece of clear packing tape, wrap it around my index and middle fingers a few times, then stick the spacer on it and press against the spinning sander with off center pressure...
...which gives me an idea for accomplishing the same effect with a Dremel tool...
Now, I haven't actually tried this because I just thought of it while reading this thread, but it just might work...
[li]Take a 3 or 4 inch long piece of packing or duct tape and fold it over on itself, and press it flat so you have flat pad of tape that's sticky on both sides.[/li]
[li]Stick down on the top surface of 1 or 2 inch thick piece of wood so the edge of the tape pad is flush with the edge of the wood [/li]
[li]Stick the spacer down on the tape pad so its edge is also flush with the edge of the tape/wood. Positioning the wedge at the edge of a wood block like this will give you the height clearance needed for the next step.[/li]
[li]Put a carbide cutting disk on the dremel (flat black cutting disks that look sort of like carbon fiber and usually come about 5 to a pack)[/li]
[li]Rather than using the thin edge of the disk to cut, using light pressure, press the top, flat surface of the disk down onto the spacer at a slight angle (to create your wedge shape). In essence, using dremel like a miniature, upside-down palm sander.[/li]
[li]If the disk radius isn't large enough to contact the entire width of the wedge, you can sand one half of the wedge a little, then the other, going back and forth while eyeballing it to make sure you're keeping the flat of the disk parallel to the surface of the spacer so as to finish with a flat surface on the spacer rather than a rounded one.[/li]
The only other option I can think besides this that might work when limited to a hand saw, sandpaper and dremel is to use the saw, dremel, and some scrap wood to make some kind of little sanding guide (e.g. sort of like a saw guide) that will hold the space and allow you to run a sanding block (never use the sand paper without wrapping it around a piece of wood or you'll never get a flat surface) over it at a consistent angle.