I'd say either a 1x1 or a 3x? Public work that's a bit more artistic (the topiaries would work but only from weeding day) so that something shows through the centre of the arch, then surround it with flowers. I think it'll look like a classy garden with a walk through entrance.
a bell might look nice, surrounded by hybrids? (by bell i mean the pwp) as close to the fence as you can get it so theres room for a little flower path.