I tried running my Hypercube 300 without any sort of way to manually level the bed, but ended up with the rear running about 2 mm lower than the front.  That's a bit more than Marlin's unified bed leveling can handle, which makes it difficult to get consistent first-layer quality across larger prints.

This is a T-shaped frame that supports the print bed at the front corners and back center.  M4 screws through blocks mounted to the bed-frame extrusions allow the bed to be leveled.  I used repeated UBL cycles and OctoPrint's bed-level visualizer to get mine dialed in fairly well.

The frame is broken into pieces so it can be printed on any printer with at least a 200-mm bed.  Since it's supporting the bed (and its heater), you'll want to use a material that can tolerate anticipated temperatures, such as ABS or ASA.  With 8-mm brims and ASA, I had only minor warping on one piece that didn't affect assembly.  After printing, assemble the frame with an appropriate adhesive or solvent (I used acetone).

Both STLs have all the needed printable parts, oriented optimally for printing.  M4 4-mm heat-set inserts and M4x35mm screws are needed to complete the leveling blocks that are mounted to the front and rear extrusions.  

The bed is intended to sit in the frame and needs no attachment or screw holes; the weight of a quarter-inch slab of aluminum tooling plate is enough to keep it stable.

bed_frame.scad includes a preview mode with a visualization of the Hypercube 300 bed assembly (at least as I built mine) that relies on files not included here.  Check out https://gitlab.com/salfter/hypercube-300-remix and view it from the bed_frame_v3 directory.

Printed on my AM8 with 3DXTech red ASA (T-frame) and MG Chemicals red ABS (leveling blocks).  This bed replacement was an exercise in getting ABS (and then ASA) to not warp.  At first I was printing longer pieces, but in ABS they'd start warping fairly quickly, even with a 10-mm brim.  ASA tended not to warp as much, but the big pieces were still usually curled up at one end near the end of the print.  Keeping the length of most pieces down to less than 100 mm (and orienting them vertically when possible) almost completely eliminated warping when printed in ASA with an 8-mm brim.