The Zentrans and Meltrans were biologically engineered to do whatever job the Protoculture needed from them....thus the wide range of sizes, colors, and flavors... You don't modify the mecha to fit the soldier...you modify the soldier to fit the mecha....
With that said, we would just need a figure that fits the cockpit of the HMR Regult and it would have worked if Bandai had not built the details inside of it in a scale that doesn't match the rest of the Regult...
it's like the Regult is 1/100 but the cockpit details are 1/72....in a way it is Bandai's nod to anime magic!