A modified 10 inch, carbide trimmer blade, adding to it six M6 one inch long bolts with their corresponding nuts and washers, nut-driver needed, and two 9 inch long pieces of trimmer line, as per the pictures below. You may use any diameter trimmer line, but I prefer the thicker 0.155" trimmer line, for the line pieces will last a long time before needing replacement . Thinner line brakes more often, depending on how you use your trimmer. This modified blade can be attached to a gas trimmer, 25cc or higher recommended, using the blade add on mechanism recommended by the trimmer manufacturer, and presto, you have turned the trimmer into a super-powerful brushwhacker.
The trimmer blade, bolts, nuts, washer, nut-driver, and trimmer line my be bought at eBay, Amazon or any other store, online or offline, that sell gas trimmer parts. If the blade has no holes for the bolts, you will have to bore them yourself, easily done if you have the proper equipment. [blades are made of hardened steel]. Carbide tipped trimmer blades with plenty of holes can be purchase at eBay for less than 15 dollars, shipping included. There is where I got the one shown in the pictures; that one is old, I bought it brand new a long time ago. It has cleared over a mile of overgrowth, and have lost most of its carbide tips. Even this old, it works great. The teeth can just be resharpened with an angle grinder. But if you want to look sharp, a brand new, shining blade, cost less than 15 bucks, shipping included.
Everything wear out over time, so order enough parts for replacements and to make extra modified blades for you and your friends. Once they see you using your modified trimmer blade, they will want to buy a couple. Be ready to please them, and make a few extra dollars in the process.
PLEASE BE CAREFUL, FOLLOW ALL SAFETY PRECAUTIONS WHILE PUTTING TOGETHER AND USING THIS MODIFIED TRIMMER BLADE. THE DAMN THING IS POWERFUL. DAMN POWERFUL. MAY BRAKE YOUR TRIMMER AND BADLY INJURED THE USER AND THOSE AROUND HIM/HER. SAFETY FIRST. THIS IS NOT A TOY. BE CAREFUL.