How To Build
First you will need a Linux box, I used to use Ubuntu but lately I'm using Synology box for building so it is possible but you will have to make some manual fixes. In any case you will need Python3 installed. For actual package build I'm using Synology scripts.
Assuming we are building for bromolow platform and DSM version 6.2 following commands need to be executed.
At this point you should have something like this.
Next we need to build our package, so from toolkit folder we first need to build Linux kernel which is required for building virtualbox drivers, this is executed only once.
After kernel we can build virtualbox package.
If you made some changes to virtualbox package just rebuild vbox, kernel doesn't have to be built again.
After second command you should check if you have spk package.
If not check log.build and log.install in build_env/ds.bromolow-6.2/ folder to see what went wrong.
Synology build
For building on Synology box you need to install python3 package and create link in right location.