Before I did what you are considering, I'd need to know what pressures the gun is safe to shoot, what pressures the shells you plan on shooting are, and then I'd consider making the gauge reducers from a high tensile steel. I'm not all that familiar with what the common commercial Gage-mates are made from. If from aluminum, I'd sure give some thought to make some from steel. I'd also run the numbers to understand the stress exerted on the barrels from the 20g shell without consideration to the strength of the Gage-mate. Any added strength from the Gage-mates I'd just consider as additional safety margin.
