This seems overly complicated to me. Since you're planning on buying a 48"x96" piece of plexi anyway, if you were to buy another small 24"x24" sheet for a bottom piece you could make a standalone 24"x24"x48" tank and use your existing exo terra as a quarantine tank for your new addition, then sell it afterwards if you really have no use for it (or get another species of frog).

I don't have any experience working with plexiglass tanks though, I do understand it can warp if you aren't careful with heat sources so I'd probably just make the larger tank out of glass but it would be pretty heavy.