I made a DIY canister once... It worked (and didnt leak) but wasnt practical at all.
I used a small fountain pump that i had lying around. Unfortunately it didnt have the head to pump water from below the tank. It only worked when setting next to the tank.
So make sure that 20-80gph pump has the head that you need... and remember the higher the head the lower the flow.
Also priming was a pain, their was always air in the canister. Not sure how to incorporate a fix for that. Tubing and a syringe maybe?
Opening the canister without making a mess was impossible. the canister and all tubing had to be carried to a sink..... ball valves and quik connects would almost be a must if your running tubes behind a desk.
Final thoughts: It was a fun and challenging build. Although it worked, didnt leak and did the task - the practicallity of it was a failure. After hours of time, spliced wires, ruined containers, wasted sealant, and tubing I should have gone with a Tom Rapids mini - id probably still be using it today.
Although Im an avid DIY'er I honestly think canister filters should be left to the professionals. However im not saying it cant be done.....