Draw out your hole but then cut it several mm or an entire cm too small. I use just the dremel metal cutting disks which work well on abs and fiberglass+epoxy. Just it melts the edges a bit on the abs.
Then get some smaller precision files and ONLY use those the increase the hole size. Constantly check with the heatsink and VESC attached to see if it’ll fit. File down more as needed until it barely fits in. I want to say I had 4 mm or so left to remove when I switched only to the files. Before that I did use a grinding stone to remove smaller amounts at once.
You could risk cutting the hole bigger with the cutting disk but I’ve found that the uneven edges left over usually make it to where you have to make the hole too big if you want it to be a clean cut.
This hole is so tight of a fit that it can hold the focbox and the three separate heat sink plates it has on it up without me bolting it in.