There are such fixtures out there, but 'can be' expensive. The question is how do you keep it from moving on the timber? If it's clamped down, then it's similar to the Makita. You probably can't use your weight on it like you would sit on a boring machine. Not sure though. I have seen one or two "drill press"-like fixtures for drills, but never really examined them.
Carlmill, have you ever used the Makita or a boring machine? Where are you located? Maybe you can try out another members machines to help you decide? New Makitas are $1305 plus shipping last I checked, but used ones go for less. I bought a used one with an extra chain for $1000. Used ones occationally on ebay or craigslist, but members often list used ones for sale too here. My boring machine was about $150 and good bits are $25-$45. Jim Rogers can help you with a boring machine and bits if you choose that direction. He posts his tools for sale in this section. I've bought a bunch from him, good guy and good tools. He doesn't sell junk. And everything I've bought from him was sharp!