A way to keep my cat off the top of my terrarium?

[pretends2bnormal]

I don't own any cats, but from friends who do have cats, heres a few of the things they had some success with for keeping a cat off of counters. You can treat the cage top similarly, I expect.

Strips of tape, facing up, along the edges and in stripes across. No need to block 100%. Most cats do not like the feeling of tape on their paws and quickly become disinclined to jump on places that hace/had it. Ymmv by personality.

Similarly to the tape, lots of cats dont like stepping on tin foil, so taping strips along the lid, not 100% coverage but enough to ensure kitty has to he stepping on it somewhere, may have a similar effect.

Definitely a good idea to re-inforce the mesh. Unless you have an exceptionally heavy cat, a sheet of acrylic with holes drilled/melted into it for ventilation (however much you need shouldn't prevent structural integrity I think). Just make sure it isn't resting on the mesh, you will need the glass edges to support the weight.
Glass can hold more perpendicular weight than it would seem. Think about how much weight glass lids are for smaller aquariums made out of similar thickness glass to the exo terra tanks.

Last resort, could you mount a cheap shelf above the cage that covers the depth of it only a few inches over the top? No ventilation really lost. May be tougher if you use taller lamps for heat or light, but if the gap is too small for the cat to get under easily, it'll be the most effective, I would think. (And look, storage!)

I dunno, just some ideas. Out of people I've known with cats, the water bottle thing works in much fewer cats.. many only obey it when it is present or you are, some don't care, others flee from it when it is out, but disobey anyway until they're getting sprayed. (This is across 20 different cats over the years that started with it at varying ages, babies up to adult adoptees)

Good luck!

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(Typo, cars does not equal cats, lol)