Hello Ikea expert, thank you very much!
It's quite something that the Maximera drawers are 8.4 cm narrower inside than the cabinets, which wastes a lot of usable space, and 6 cm less deep! So their usable base area is only 48x54. It's a pity that you don't have anything from Förvara, they are constructed differently and you don't know how much width is taken up by the side panels there. I prefer Förvara precisely because the bottom there is not curved.
I want to put together my own trash system because I find the IKEA bins too small. With the amount of recyclable waste that always accumulates, they are by no means sufficient, and that's why I planned to make a pull-out trash box from a medium Förvara drawer and a 65l SAMLA box. But unfortunately, it's 56 cm too wide. Yeah yeah, IKEA really makes sure that you can't combine things but are forced to take their more expensive specifications...