Your setup sounds good. I think the coconut coir might retain too much moisture, so just try the sand & soil mix I think you could get by with 3". When my torts dig in they are often happy with just being partially buried. 5" of sand is really heavy and may break through the tub.

Here's a company that makes larger tubs for not too much $$$

www.reptiletubs.com/

Look at the VE series tub. It's about 52x24x14.

Also look up vision tubs.

good luck.