I just got this quick idea to see if classic CR-10-style heatsinks would fit such that it can still be easily mounted onto a CR-10 / Ender3 whilst using the 2x M3 bolt mounting points (and not having to print a holder). This is because I was thinking of the Coaxial8or from a finance perspective.

6 inputs?

The main question I expect to get is "Why 8 and not 6 inputs?" and one of the reasons is that, financially speaking, the difference is about a £30 upgrade. The cheapest option to run a 9-axis Marlin configuration is an MKS Monster8 + BTT EXP MOT and it's probably cheaper to buy 10pcs couplers than 6 individual ones, thus the only difference in price is 2 of the following:

• Nema 17
• Extruder
• Stepper driver
• Dragon heatbreak

Additionally, there exists "flat point" grub screws. 10pcs M6x6 only costs £2.75. I'm assuming that if the heatblock is first filled with cleaning filament, the unused channels stay inert and don't clog, allowing for the possibility to upgrade to the full 8 inputs in the future.

For the aluminium print itself, the auto-quote is $46 and I doubt the ~5mm shavings in the X and Z bounding box would account for much, since it seems that the minimum PCBWay cost is around $38-40.

4 inputs?

The financials (and ease of set-up) for 4 input make more sense, as the heatsinks are cheaper, all the drivers fit on a single board and could even be a pseudo-drop-in replacement for anyone using a Zonestar 4-in 3D printer / Prusa MMU1. 

A 2x M3x35 bolts would be needed to mount the hotend, and a 7mm spacer would need to be bought or printed:

The result is what I'm currently going to call the Coaxial8-4AB:

I believe it might be easier to ensure a sufficiently tight seal with these normal heatbreaks because of the PTFE gasket.