Just to add on to this, a lot of people swear buy this;
https://sites.google.com/site/hpftechllc/home/hpf-preIt has some stuff in the manual in the DIY section about a "butterworth filter" and some other stuff I don't understand :p.
It might be a good place for inspiration. There is a schematic there, but getting it onto stripboard might be a bit of a nightmare for me to do.