How thick would you say the base of your concrete is?Yes, those privacy strips might help Opo not see over where the grass is GREENer! Yep, thoseTrex boards will work well & never need painting or staining.
You could just dig a hole the size of your large plastic tub, dig out the dirt & fill it up. Not sure if you want to fool with a shallow cement wading pool.
Take a look at the wading pool here —> https://www.tortoiseforum.org/threads/new-turtle-pen.181089/