Indeed, I wondered there is no instructions supplied, I lately received additional package of m2 extenders and there is no instructions how to install too. However, everything seemed logic to me and cables were marked with blue on upper side which kinda made sense but still, you cant know it and I agree, instructions should have been added, something like qr would even save on paper. What I wasnt sure about is why 2 cables were delivered and if they are different, so I tried my luck, luckily nothing is damaged.
I did place it under rockpi board, if somebody says he requires better access and places it over cpu heatsink to gain better access to SD slot, then I would hope RADXA to add some kind of a heat shield beetwen the heatsink and m2 board. However, I preffer to split them as far possible from each other as they both produce heat, I actually placed the upper part of acryl housing between rockpi and m2 board, however, I had to use additional bolts etc because there is not enough for such construction delivered. As for SD access, I do not think that I need it more often than few times when I reflash or recover because internal emmc or disk are broken. So far you can place SD card, if you have tweezers, you can use them to place SD card easily into the slot.
If somebody bought acryl housing, then one (upper) side is unused due to the huge heatsink, I hope RADXA will consider adding few bolts more either to m2 package or rockpi4 housing itself.
EDIT: And yeah, if you look up picture 4 on wiki guide, then it isnt actually how I would like it to be, I would like if heater is on lowest level heats everything up and if you flip it, you place full weight on your M2 which probably is also not good, thats where I place and used acryl housing, lower part of it is first, then there is enough room for M2 board on which are heatsinks for M2, then the upper part of acryl housing, then rockpi board itself, then big heatsink, will maybe make a pic when I am back home.
One additional thing, bolts etc transfer the heat, maybe using different materials would end in less temperature on device.
EDIT3: here a pic of one device without m2 disk
If there is SD extender cable (which there is) and it is long enough (which I think it is) then it can be moved between m2 board and acryl to the opposite for best access as your device is mostly turned to you with USB side, which would bring SD to the same side
EDIT4: this is another device, where the cable does go differently and with this construction, access to SD is slightly worse than the one of previous device, just due to different cable placing, you can see inserted SD card and M2 disk
Here you see that there is enough place for heatsink for M2, even for those delivered with acryl casing (as I had huge heatsinks, small were delivered with acryl case, so I though, why not using them to cool down M2 from my old notebook which was in notebook without any cooler on it:
EDIT5: M2 extender board cant be properly placed if there is eMMC card, it must be done gently, otherwise it could break by the pressure caused due eMMC card being higher, which I bet many users would be confused if they do it right, especially as there is no guide and no pic with installation when eMMC card is also installed. With that we would be back at point of criticism that main installation guides included only steps how to flash with some third party instead of how to flash directly from SD card to internal emmc (with dd as example which works on rock itself, not like suggested soft which doesnt), because if I have to take out emmc, I have to reassemble whole device, which is nasty. Last point of criticism connected to it would be RADXA not providing offline image for internal flashing, as for that it is enough to simply add compressed image into the image itself which would simply result in double size of current image but would provide everybody ability to flash devices right after they prepared their SD card. For me and my releases, I included all flash scripts as well as things like autoflash of m2 disk on first boot if FLASHM2DISKDEFAULT exists on first boot, that way installation and setup of rockpi is for me very simple, regardless of where I want to install it, I described it without to many details here: Installing images with DD, no need for emmc adapter, run on USB3 or M2 per default