Slices of pre-painted Tamiya masking tape, or perhaps 'Sellotape'-type clear parcel tape, again pre-painted. Should be OK in 1/48th scale, especially as it's going on the underside, so less shadows to cause it to look thicker than it is. The other option is to ensure the relevant panel line is filled, then mask and paint the 'tape' strips.
I thought of painting, but it goes over top of a roundel. I may have to use the decal. I think it'll look ok on the lighter blue as opposed to the DSB.