I've been modifying my Rostock Max V2's Onyx heatbed with an aluminum heat spreader above and extra insulation below.
The heat spreader frees us up with respect to how flat the slightly flexible Onyx bed has to be, while the extra insulation necessitates some changes to how the bed attaches to the base. After fiddling around with bolts and washers for half an hour or so, I threw my hands up in the air and said there has to be a better way to do this. Here it is.
These arms are 150mm long on center, the same spacing as the Rostock Max V2 bolt holes, so they should be able to sit nicely together underneath the bed and provide a clear guide for the screws attaching the bed to the base to go through. They're 10mm tall, so a little more than twice the washers the Rostock came with; I'm estimating roughly as tall as the original washers + "snowflake" spacer.
Material is probably not super important. Higher temp and compressive strength material would probably be better, but I would expect PLA to work. As far as I know the washers that these replace are nylon, so that might be a good bet, although I'd expect ABS, PETG, PC, PEEK, etc. to work as well. I'm planning on annealing mine.
I printed with support everywhere without thinking about it and it turned out fine without too much extra time, so I'd probably recommend doing that. A printer with good bridging may be able to do without though due to the fillets.