Look online for Flukers perches. They make an adjustable perch that is very nice.

Aside from that, you can always silicon on those plastic closet pole holders, then add your own dowel perch. A dab of silicon will hold it in. All removable when needed using a razor blade.

And if you are brave, you can get a glass drill bit and attempt to drill screw holes to mount a perch that way. Those perches they sell for birds are nice. I saw a glass tank that someone had drilled for that. I've never attempted it myself.
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Mark