1rst build, time to do next step

[JAZZCNC]

the bed pulls out like a drawer and can have a few mm error

No BAD idea it will resonate and will show in the quality of cut.

You don't need to make the bed tight fit just have few mm gap and put the angle brackets so they fasten to the insides of steel uprights. Make the side guards out of flexible rubber or plastic like Damp course fasten to bed sides and at top rail so it travels with bed and the brackets will be underneath out of way and accessible from out side.

Then you just surface the sacrificial board on the bed to bring parallel to machine.