They are more likely to breed in a large enough, properly set up established 'container' outdoors if temperatures are between 70 F daytime and 50 night. By now you might not get a successful spawning untill fall when the temps drop and then you'll have problems raising the fry as temps continue to drop. The gold fish will 'appreciate' the outdoor summer vacation, though. You can set up the container and then let it just rest over the winter, if a freeze won't damage the container.There are some hazard outdoors that you don't have indoors. Predators and heat for two. It can be more trouble than it is worth. You have more control of conditions in the same large container indoors. If you are just moving a 10 to 20 gallon tank outside forget it. Commercial breeders do it indoors, or have a large enough operation they are willing to take the losses outdoors. Notice 'properly set up', 'established, and 'large enough.' The bigger the container the easier for it to be 'self establishing,' 'self maintaining,' temp variations are also lessened. The depth should be at least 2 feet but with shallow areas of no more than 6 inches for the fry.You can do that by putting washed bricks, rocks, pots etc in it.You can hide the bricks with stone work ,plants, gravel etc. if you need too. Give a lot of thought to where you are going to put the thing. It needs some sun but not too much. Morning sun, then dappled shade, as much as 50-50 sun to shade, in mid day then open shade the rest of the afternoon is ideal. Certainly no direct sun after noon. Install what ever filtration and pumps you want, you don't really need much filtration if the thing is big enough, and you definitely don't want a powerful system that is gonna suck up the fry, some circulation is good, even if it's nothing but an airstone , fill it about half full of water and let it age and settle for a couple of days. Now add your plants. You want live plants, they will act as natural filters and without them so much algae will grow that the water will turn dark opaque green. Don't try to plant them directly in gravel at the bottom, defiantly don't try using mud, the fish will have that constantly stirred up. Use regular old clay flower pots, put a layer of coarse gravel in the bottom, then some finer, you can add a layer of garden soil then the plant but make sure it is covered by several inches of gravel and put the plant in there. Put the pots in the container, on bricks to raise them to the right height and disguise the pots if you want. Some floating plants with dangling roots are needed to catch the spawn and give the fry a hiding place. There are floating nylon "mops" that are sold for the purpose and some of them are disguised as plants, but Water Hyacinth is ideal. BUT Water Hyacinth is illegal to sell or even own in some locales as it is an uncontrollable water choking pond destroying pest.Fill it on up with water and let it stabilize for a week and then add the fish. Feed the fish as you usually do but take extra care not to leave uneaten food in there. Floating food is best and remove it after at most twenty minutes. As time goes on you might not need to feed as much. Now just wait. If the fish spawn they will do it in the morning spreading the eggs into the floating plant roots or mop where they stick. At 70 deg they should hatch in 3-4 days, they are way small so look close. If your container is large enough and has been set up for a month or so there should be a supply of natural food, unfortunately conditions for a supply of food are the same for a supply of predators, but again if the thing is large enough some fry will escape. 2 days after hatching you can feed the fry hard boiled egg yolk, crushed extremely fine, and just a small pinch right where the fry are. Careful not too much too often, just one pinch about an hour or 2 apart it will make a yellow cloud. Commercial liquid foods are available. 2 weeks later you can use baby brine shrimp, 3 weeks- fine powdered food about 3 times a day. Over the next 4 months you should be on a 2 times a day schedule while gradually increasing to adult size food. When I was a kid ( in those days we had to make our aquariums out of stone using only flint tools) I had 4 refrigerator liners that I found at a local dump. I plugged the holes with bolts and washers and silicone and all kinds a stuff. They aren't very pretty but they ranged from 300 to 1000 gallons! They were basically free too, instead of $800 + for an aquarium. I can see setting one of those up and putting up some kind of screening wall, even masonry, with a bench on top.Now I use a cheap kid's stiff plastic wading pool about 5 to 6 feet diameter and 2 to 3 feet deep, around 600 gallons. ( the pool is cheap not the kid) There are even larger ones. You can set that on the ground and put potted plants in front, I dig a hole a few inches shallower than the pool and bank soil up to it and plant stuff, even just grass on the slope. I use spray cans of browns greens some blues to abstractly cover the cartoon decorations on the rim and a few inches down the inside, deeper than that gets covered in algae. Just use a paint that won't dissolve the plastic and let it dry good B4 adding water then a few days B4 plants and fish. If somebody asks I'll give full details on the cheap kid's wading pool backyard pond. In this case the kid is cheap and it's me, as details include where to get ultra cheap, even free, fish,food, and plants.