This is a bed support for Kossel XL printer, designed around an Onyx 310mm bed heater with twelve M3 screw holes and 360mm 2020 horizontal extrusions. I provide OpenSCAD source files so you can tweak the design if desired or necessary; but beyond a certain point it may be better to simply find another bed support.
To be useful, you'll need to print three of these Things. Each requires four M5x16 screws and four T-nuts for attaching the support to the frame, plus one to three M3x12 or M3x16 screws (depending on how thick your bed is) and nuts for mounting the bed, and matched M3 wing nuts or regular nuts with printed bits to hold the bed down. If you like, you can add springs and washers to enable physical bed leveling.
The spacing is very tight around the bed screws. During assembly, it helps to not fully tighten the bed support to the frame until after you've fitted the bed. Although the geometry provides very little "wiggle room," there's enough when the support is not fully screwed down to enable some fine-tuning of position. You may also need to straighten some of the bed screws; they don't always go in quite straight. I used two screws per vertex, for half the Onyx's twelve screws. Using three per vertex would probably make it too difficult to fit everything in place. One per vertex may well be adequate.
This Thing is designed to be used with the Kossel Mini 2020 Frame Brace (https://www.thingiverse.com/thing:1686378), but can be used without it.
I printed mine in PETG with the specified settings, using my Robo 3D R1+ to print parts for my new Kossel XL. If you'll be using this bed support with a heated bed, be sure to use a plastic that's resistant enough to temperature to handle the bed heater, and/or add insulation between the bed support and the heating element. The "clear" PETG I used shows a few bits of charred filament here and there, unfortunately.