Vertical Dropouts

First, I know there are a few threads on this theme around the forum - I have read them, and hope that this post doesn't upset too many people by going over covered ground.

Right, to the issue... I've just finished doing a fixed conversion with the aim of creating a city runaround that won't look too desirable for someone to steal, but is secretly a lovely bike. However, the frame has vertical dropouts with almost no wiggle-room whatsoever, so I've had a really tough time getting the right chain tension.

Using a half-link I've managed to get it to a rideable tension, but any indecision or sudden change in cadence and the chain has a tendency to fall off - so I need to do more work to tighten it up.

I don't want to buy an ENO hub unless I really have to, and want a solution that's a little more flexible than finding a magic gear ratio - so the only other option I've been considering is filing the dropouts a bit. However, I'm really nervous about doing this, so thought I'd post this thread with a picture of the dropouts just to check with you guys that it would be OK to go ahead with this plan.

Taking the extra half-link I've used out again makes the chain too short, and with it in then it's slightly too long - so I'm assuming that I only have to move the axle back maybe 8 or 9mm to fix the problem. There should be just enough room for this (see attached picture), but I'm really concerned that this will cause the dropouts to fracture in the near future. What do you think? Is there anything I've overlooked that could be problematic in filing these back?

Also, is there anything else I could do instead that would be less destructive?

Thanks in advance for any comments - I've been viewing this site for a while, but this is my first time asking a question.

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