You will need:
1 #10 Bung - this is your standard rubber bung that will fit a Better Bottle
1 3/4" ID - 1" OD Vynil tube cut to length - make sure your tube is 1" OD, as Home Depot has failed me in the past with OD measurements.
1 1" Spade Drill Bit, it looks like this
1 Impact Driver - make sure your spade bit is the correct fitting, usually 3/8"
1 Table Vise Clamp
1 surgical mask
This procedure should explain itself. You are using a wood spade to drill a hole 1" through your bung, which when drilled through will fit a 3/4" tube as a blowoff. It is absolutely necessary to use an impact driver as a conventional cordless drill will not facilitate enough torque to do the job.
Tighten the bung onto the vise, which will contort it's shape. Use your free hand to push the other sides to make the bung look more like how it would normally. Slowly impact drive through the bung; it will take 5-10 minutes.
This makes rubber ash fly everywhere - use protection and cover your mouth and wear an apron or slack clothes.
The hole made will not look pretty but it will be structurally sound and the tube will fit in with some effort.
This project will not save you any money over buying that whacky PVC blowoff kit seen online unless
you already have these tools. And since you likely have the bung already, the 4' of tubing set me back just under 5$, just over 5$ if you want to start from a new bung (I used a predrilled bung just fine).
NOTE: I still would suggest an impact driver over a big plug-in drill or hammer drill as you need one free hand to hold the bung in shape, unless you want to use a large hand vise grip to hold it, but I think that would be putting down too much power... but it's worth a shot, I suppose.