Depending on the complexity of your mark (in blacksmithy, it's referred to as a maker's mark or touch mark), you can get a scrap piece of steel and use a dremel to get rid of the negative.
Most steel shops will let you poke through their drops, and often they'll just give you the little bit you need. Have them, or someone, weld a piece of pencil rod on the back side. Turn a handle with a 1/4" hole for the pencil rod and voila.
For my mother's, I got a piece of 1/2" x 1" bar stock, ground and filed the end flat, then cut a thin slice after dremel-ing (is that a word?) the mark. The 8" bar will probably let me make a couple dozen marks, if needed.
Don't forget to reverse the mark, if it's not symmetrical - I had my mother's completely finished and tried it out before I realized I forgot to reverse it.
Also, try to minimize the 'extra' steel around your mark - this will let you heat it much quicker with a propane torch.
When using the iron, I told her to hit the back side of the brand with some steel wool or fine sand paper, just enough to get 'shiny' metal. Then she heats it with a torch until the shiny bit turns blue before applying it to the wood. The oxide color is an indication of temperature, so this allows her some consistency in the way the iron burns the wood.
I don't have any handy (I'm goofing off at work) but I'll post pictures of the one I made my mother when I get home this afternoon.
I still haven't gotten around to making my own, even though my mark is far simpler than hers...