if this is something u really wanna do, just use another piece of the plexiglass u are using for the top to use as an interior wall. use hot glue or a similar adhesive/sealant to divide it off. good luck.
as for a hygrometer/thermometer? try this by Begrit. it's like $12 and has a 4.5 stars rating on Amazon. imma order me 1 or 2.
can I remove it later? Will it have any residue left once I remove it?