The Colfax lowers have the top shelf already cut, so their jig doesn't cover that, but it's not hard to do without a jig. Look at the engineering drawings and measure carefully.
I've done a few with a Dremel. Most recently my son gave me a drill press for Christmas, so I used that. Full cost was about $90.00 at Harbor Freight. I suspect he paid less, 'cause there's always 20% off coupons floating around. A box of mixed size end mills at HF cost $31.00 on sale after coupons. Digital calipers were a couple of dollars. Worked just fine. Had to smuggle it into my apartment. Landlord said I couldn't have a machine shop.
I'm sure others have said it, but i'll repeat-You will spend more money doing an 80% than you would spend to buy a stripped lower. These are for fun, or to stay completely off the radar, not economy.
FWIW, Colfax is sold out of pre-anodized lowers ATM. I'm on the waiting list, they say a few weeks.