Fiddly bits of wood on iron

I've recently been attempting to renovate a garden bench that we found in the vergeside chuck-outs a couple of months ago. It's a nice cast iron and timber thing, and I've slowly been brushing and filing the iron and dressing and cutting the replacement timber (my dad found a terrific lump of wandoo that he ripped up). The first part of this went reasonably well, and I made a good square mortise-and-tennon back board thing, but now I'm up to making the ends of the boards fit well against the metal tabs that the bolts go through — and it's proving a bit annoying. I keep giving up, but will have another crack at it this afternoon.