Universal spline question

I purchased 59-067 drop in spline. wrong size, does not fit into slot, I need something for a 1/8" slot in a Larson storm window in a closed in sun room. I need to replace some of the glass in the windows and was going to replace the spline also. I just noticed the universal spline on you web site, and was going to try that. Would it work? The 5/16" was way too big.