The aluminum carriage that comes with the i3 is way too thin. It's a shame they saved $1-$2 at most.

Before replacing the heated bed, replace the carriage first. There are no direct replacements for the i3 that I have found. Most people get one for a typical Prusa i3 and drill additional holes. I just got a 12"x12" piece of aluminum, which was cheaper than one that I would have to modify anyway, and a friend of mine with a handheld grinder cut it to size and used the original carriage to drill the holes for the bearings. Don't worry about cutting triangles out of the middle or cutting metal from around the corners. Those serve no purpose and just make it more succeptible to warping.

After you replace the carriage you likely will not have to level the bed again for weeks, possible months later. I replaced mine a little over a month ago and after the initial leveling, I haven't needed to level it again since and have done at least a couple of dozens prints.

The piece I got from Amazon.com: just enter 639725466794 and it will take you to the item.

For ensuring a perfectly level bed, get a piece of glass for the bed. It's difficult to do, unless you use a heat gun, but I removed the buildtak sticker first and cleaned the glue. I had gouged mine using the putty knife and it looked ugly. Though some people leave it on.

For best results get some heatsink thermal pads to place between the bed and glass.

Besides having a perfectly level surface, the glass can be easily removed if you have a stubborn part to remove. And you'll want a couple of binder clips to help keep it in place. If you have seen pics or video of people who use glass, some will put clips on every side, with a couple of clips on each side. I found that to be useless. With the thermal pads (which are just sticky but not adhesive), I use ONE clip to keep the glass in place. Then one clip to remove and take the glass off if needed.

And what has worked best for me is cheap glue sticks from the dollar store. It's been a dream to use since then. Hairspray would be a good alternative and since you can easily remove the glass you won't be spraying hair spray around the printer, which makes me cringe when I see it in Youtube videos.

Some people use regular glass by getting a $1 picture frame from the dollar store. I've seen people use a mirror which is kinda cool. But typically I read that it's best to use borosilicate glass because it can handle higher temps if needed. So to be safe I went with that.

The piece of glass I used that did not require any cutting: search 6922302513160 on Amazon.

The best thermal pad to use: B007PPEW52 on Amazon. It's a huge sheet, enough for 4-5 (at least) uses. There's no right or wrong way to use. But you don't have to cover the entire bed with it. And don't cut out a couple dozen small 1 inch squares and think that will give better coverage. The thermal pads are made to transfer heat very well. The glass is a heat conductor as well. 3 rectangles about 5-6 inches a piece (just eyeballing mine right now, again not that critical) works great for me.

The thicker carriage and glass is the greatest upgrade I have done on my printer and took the frustration of bed leveling away so that it made it SOOOOooo much more enjoyable to use the printer.

<sorry for any spelling mistakes, don't have time to proof-read>