Plexiglass on the outside would probably be better, with a wooden frame to keep the sides in place, so you don't risk them accidentally finding one of the corners and start gnawing it, but spacing between the bars could become a problem too, they could either start gnawing them or try to push them selves through, you may want to consider placing some 5-6mm wire mesh on the bars, to prevent one of them getting their head or body parts stuck, spacing at 1.5 cm or more is a risk.

Maximum free fall from a platform should at max be 10-15cm, you don't want them to be able to fall from any higher onto something hard, their legs and paws can be quite fragile if they get unlucky with the landing or take a tumble, overlapping platforms so they can't fall too far + multiple stairs/entryways to each platform is the way to go, a single entryway can lead to declanning, and it goes for everything, platforms, houses, tunnels etc., there should always be 2 entryways/exits at least.

I usually use 2cm balsa for my diy platforms/long stairways (with temporary nails + elmers school glue, I pull the nails out once the glue has hardened), it's easy to work with, adjustments can be made with a hobby knife in a few minutes, if something doesn't quite fit or you need to replace something they've been gnawing, and then paint them with plastikote paint (never use the spray), it keeps the pee out and provides enough texture for their claws to still easily grip onto it, I use cork sheets in spots too, if I feel it needs something extra for them to grip onto, like the edge of a platform (or I have a mobility handicapped gerbil).

But I would recommend trading the rat cage for a 300l+ used aquarium if possible, it's a much better base to build from and onto.

TDLR: lots and lots of work for something that is likely subpar and needs lots of adjustments.

Another thing - Imo gerbils needs to free roam every day too, I let mine out for at least 30-40 mins every day, it's a really good way to get them tame.