DIY sump for 75g goldfish tank, baffles spacing?

[GoldFishGal]

So I'm upgrading my goldfish from a 40g to a 75g. They're getting huge, like proper chonkers lol. I already have a stand (built it with my uncle, will share pics later) but now I'm thinking about filtration.

I want to try a DIY sump because canisters are expensive here. Got a 20g long for the sump, some acrylic sheets for baffles. But I'm stuck on spacing. How much gap between baffles? I've seen 1 inch, some say 2. Also, what silicone to use? I don't want to poison my fish.

Also, I was reading this thread about canister vs HOB vs sponge filters, and now I'm second guessing. Is a sump overkill for goldfish? They're messy but maybe a big canister is enough?

https://aquariumowner.com/threads/canister-vs-hob-vs-sponge-filter-which-is-best.16

Any of you bhai log built a sump before? Need some gyaan please 🙏