Doesn’t really matter about perfect fitting, in fact the slight oversize makes it far easier to make sure all is covered and also increases a bit of thermal mass.
The heatsinks are so cheap that even locally 90% of cost is shipping when buying singularly and would be a great option on the product page even if $2, but a cheap, cheerful stick on would be good enough.
For slim devices if maybe you not using the standard header maybe a plate type heatsink from the mounting holes could be of use, haven’t really spent much thought but the great passive heatsink on the pi400 came to mind if you had the off board antenna. Maybe stepped with a thermal pad for ram?
Whilst I mentioned shipping as the lower the value of the product the shipping has far more impact.
Radxa always supply product in these great plastic boxes and if you go for without headers onboard with the box supplied is there any chance we could but x3 in a single box (foam seperator) with the same shipping costs?
I really like the price and actually thinking as a general purpose builders SoC I could be tempted by the x3 savings.
Lols also as a tip https://www.aliexpress.com/item/4000557509533.html usb hub works really well there are some with a purple pcb that don’t seem to work well at all but these are really cheap and good.