They are relatively small tanks.
I thought that they are all glass.
The Oceanic BioCube 29 gallon from www.bigalsonline.com is a great choice.
A 29 gallon aquarium is far too small for a stingray for all but the most temporary of housing arrangements. So, no.
You can't "Build" it from scratch. It requires very experienced craftsmanship with wood working and acrylics. However if you are referring to the kit, they come with detailed instructions on how to do so. From assembling to adding fish, to maintenance, its all there.
Usually a small, dedicated salt water aquarium is perfect for seahorses. A great choice for seahorses are the 'nano' tanks sold as packaged deals. Check out the Oceanic BioCube 29 gallon at www.bigalsonline.com , it is perfect for seahorses. I put a link to the product.