Here’s an update. It does indeed fit very well. I have very little wiggle going on. I recommend the 4mb Saturn ram cart cases for Solatorobo owners. Hope this helped.
I think I found a solution. It's not exactly perfect, but it may fit pretty close. The 4mb ram games on Sega Saturn have a very similar box dimension and the box protector for that is readily available on Aliexpress. Hope this helps your Solatorobo box protector issue or anyone else with a similar predicament.