That is an idea, but...
I've tried to build 2 canisters from diy directions, they worked, and I build yet another one using something similar.
The biggest problem I encountered were:
1- Price, at the end of it all, hoses, connections, glues, media, trial and error parts... end up costing almost double an already proven design, such as an eheim.
2- Finding the right connections and valves that will make your job of cleaning it up easy, and you might want to control the flow and remember you'd have to remove the hoses before moving the filter where you can clean and make a mess.
3- Bonding different materials, there isn't an universal glue/adhesive, bonding pvc, acrylic, glass, rubber, hoses, all of them take a different adhesive and it's almost impossible to bond 2 different materials with a nontoxic adhesive.
4- Time is money, well too much time and trial and errors, at the end of it all you'll have too much head ache for something you could have just bought at a store and never worry about it.
5-Medias... well if you want a basket to separate the medias, well you'll have to spend even more money and time.
But if you have some time and money spare, and like challenges, don't forget to make a pic journal, everyone will enjoy seeing anything being built!