I didn't realise you wanted to use modern rings! I think (but can't confirm because I haven't tried it yet) that the "nut" from a single ring chainring bolt will fit through the gap between the tab and the crank arm, so using one of those and a standard double ring bolt you should be able to tighten it up just fine. I hope this method works, as it's what I want to do with another set I have
I'll have a look and see if it works, might take me a couple of days to try it though. If it does and you need a single ring nut then let me know - I'm sure I have a spare one somewhere that you can have for nowt
maybe someone else can confirm if this works or not?