I think it would be an easy and satisfying project. But if you're doing it to save money, make sure you price out all supplies first. I suspect that you you'll probably spend about $60 on supplies (just a guess). After you know the exact cost, it's up to you whether it's worth it or not.
On the other hand, if you're not happy with the sizes or design of the commercially available HOB fuges, then that's a good reason to make your own, regardless of the cost comparison.
If you design your own keep in mind that it will need to hold a lot of weight, so you need to engineer it to hold that weight. (~8 lbs/gallon plus sand and rock)