You really need threaded bulkheads… that is a water seal on the inside diameter of the bulkhead, and threads will be easier to seal with thread tape. And you don’t want your connection on the bottom of the bulkhead to accidentally fall off and end up with your flood tray flooding your room
The only holes you should need to drill in your flood tray are the holes for the siphon and the overflow drains. Remember , you don’t want to attach too many things to the trays so that they will be easier to remove for cleaning.
The threads stick through the bottom of the tray. The connection on the bottom can only fall of if… the entire thing suddenly explodes or implodes, which if that happened, I would be fucked no matter what kind of bulkhead I used.
Fasten the elbow to the hose and just plug this through the grommet and it’ll hang there tight as shit. Until I want to remove it for cleaning or whatever reason.
I think there’s a language barrier here, sorry. I went to the garage and took some pictures to show you what I’m using. It’s double-gasket (the greyish part is the gasket), threaded, but with a press fit “option” on the inside for various accessories. Diameter of the press fit is 24mm, so no standard pipe is gonna fit there.
You really should use 3/4 inch (or metric equivalent) standard-threaded bulkhead fittings so that you can use standard 3/4 inch (or metric equivalent) pipe fittings instead of those pre-made filter pipes, because those pre-made filter pipes are only made at specific lengths, and you will need to be able to cut your overflow stand pipe to a custom length (you can of course cut the pre-made one, but it’s so much easier and cheaper to just use plain standard plastic (PVC) pipe and fittings from your local hardware store (if you even have such things there).
What is the OD of that barbed fitting coming out of the bottom of the bulkhead? You’ll want that to be at least 1/2 inch (13mm). From the picture it looks a bit small… you don’t want too much of a restriction on your drain siphons or it will be harder to adjust your inflow/outflow ratio.
The reason I had my siphons inside the tray was so that any leaks in the siphon itself would be contained, and also so that I could adjust the height of the pickup to remove as much standing water as possible. But it looks like you are going to try to make simple loop siphons just using the hose, so that should work, and might even be easier to adjust than the ones I made from pipe fittings. The only issue with making loops with the hose is that the hose will kink if the bend is too tight and create a flow restriction in your drain siphon.
Look at this photo and notice how high above the bottom of the tray the first drain opening is… about 1/8 to 1/4 of an inch… that is how much water will be standing in the bin after it drains. That much water will end up causing algae to grow over most of the bottom of the tray. It will take 2-3 weeks, but it will happen. You can place a small wedge under the opposite corner of the tray to help the water flow to the drain corner, but you’ll still have a little puddle in that corner. It’s up to you, it’ll take a few weeks before the algae is really noticeable in the bins, and you can let it go for a while before it starts to become a problem, and you can easily clean it by spraying it with peroxide and wiping it out with a rag or sponge (which is why you want removable pots) If you use a threaded bulkhead, you can add a little upside-down U made from fittings and make an adjustable-height pickup to suck out almost all the water
You said this, which means I have to use that connector size, it’s not possible to have a 13mm OD connection into a 13mm hose. I was planning on buying the larger hose, but you advised against it, so I didn’t.
Yeah, I already plan to cut the bottom slit off, so it gets closer to the bottom of the tray and I was planning on slightly tilting the trays towards the drain side anyway. I think it’ll be fine. Some algae growth is impossible, especially considering the tray bottom isn’t just a flat surface, it has a lot of small ups and downs in there where water will most likely get caught.
I’ll test it with this siphon style, if it turns out it isn’t working as intended, time to upgrade.
If you use a threaded bulkhead, you can add a little upside-down U made from fittings and make an adjustable-height pickup to suck out almost all the water 
I was planning on buying the larger hose, but you advised against it, so I didn’t. 
