The TG-16 Manual does fit..I tried with my Spatterhouse manual
Technically you can put a TG manual in a Sega card case and close the case. Does the TG manual actually fit? No, not at all. Is it a good idea to store it in a Sega case? Absolutely not.
First of all, TG manuals are wider than Sega manuals, so there's no hope of immobilizing the manual inside the case. The width of TG manuals forces them to rest on top of one or both of the interior manual-holder tabs. Resting paper on 1/4' bumps for a long period of time is never a good idea, even if you store it vertically.
Second, not only are TG manuals wider than Sega manuals, but they're about 1/32-1/16 of an inch wider than the interior of the whole case! To close the case, a TG manual has to span the interior diagonally; even then, the spine presses on the manual just before the case shuts (see pic), warping the manual. I don't know what kind of freak Splatterhouse manual you have, but seriously try putting a standard 4.75" manual in a Sega Card case and tell me it fits without warping.
I still think it's a good idea..
Let's see... the plastic is slightly more durable, but it damages manuals, it's not cost effective, it's not aesthetically pleasing, it eats storage space, there are better, cheaper options, and you can't refute any of these claims. How is it a good idea?