You doing an SBR? Most guys I know of find someone with a CNC mill. They usually have a couple of alphabet programs already set up.

The Dremel 290-01 Engraver Kit comes with a "Letter/Number Template" that apparently looks like this:

(Image from Amazon.com)

It appears to be some kind of plastic, not metal, but I suppose it would work for a while.

I'm also unsure of the letter size, plus I couldn't find the Template listed as a separate item.

Perhaps an email or phone call to Dremel Customer Service would be of additional help (for example, they might offer to send you a Template free or at nominal cost; they do seem to have a good CS reputation).

Dremel might also have other ideas about lettering guides (for example, whether the plastic lettering guides ordinarily used for drafting could be used for this application).

HTH

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FWIW, I don't think a rotary tool would work very well for engraving, with or without a template or guide. In particular, a rotary tool would be prone to chattering and/or 'walking' along the work surface − plus eating any template/guide that might get in the way.

A rotary tool excels at cutting and grinding.

In contrast, an impact tool (like the Dremel 290-01 mentioned above) is much more amenable to engraving.

If making it 'look good' is the goal, using the 'right' tool would probably help.

HTH

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Not to beat a dead horse (LOL), but if the 1-inch brass stencils you referred to are somewhat like these...

('brass stencil' images from etsy.com)

Notice that the letters are of variable thickness, or they are rather 'fat' in relation to their height.

These factors indicate the stencils are for painting, inking, etc., not engraving.

HTH

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etching machine. Temporary stencils can be made with a typewriter. Doesn't work on plastic, tho, as it requires a small current flow.