Paragon Belt dropout plug stay and brazing filler selection

I'm in the planning stages of building up a belt drive frame using Paragon flat sliders and their belt drive coupler. I'll slot the stays and use Fillet Pro but the coupler plug has me a bit confused. They have two SS sizes for 5/8x0.035" and 12x1mm. The closest pre-cut/bent stays I've found are 12.5 x <1mm. Would Fillet Pro be sufficient to wick up and fill the gap? Or should I just use straight gauge tubing that has an interference fit and higher silver filler? In general I'm just looking for recommendations on suitable steel stays and filler to use with the PMW belt drive plug. Thanks!