The missing ingredient you desire seems to be a DI box. You can purchase a basic DI box for around $50. You can then build the GBD2 and the squishy octopus and string all 3 together. You can purchase an empty rack mount enclosure and mount them in that if you wish to rest it on top of your amp.
If you wish to build your own Di box, you can do that using vero board if you are so inclined. I have made several custom projects that contained DI functions and an XLR output. However, I believe that by the time you source all of the components and fabricate the vero board. mount it in an enclosure, etc. you will have invested more than the price of a commercial DI box.