The toolbox or bracket mounted antenna won't work quite as well as one on the roof, but it does have distinct advantages (besides the obvious of not poking holes in one's roof) such as not having to pull down the headliner, easy access, etc.
That said, I'm a firm believer in taking a deep breath and drilling! Rather than a hole saw, I recommend a step "unibit"--a drill bit that makes drilling precision size holes a snap. The trouble is, a unibit is probably even more expensive than a hole saw, so that doesn't really help you.
But, for an NMO mount you don't really need a huge hole--either 3/8" or 3/4" depending on the mount--so a small pilot hole followed by the right sized conventional bit will do the trick. Even if you have to invest in a big bit, it won't be as expensive as a hole saw or a unibit.
The important thing is...DON'T DRILL THE HOLE TOO BIG! Properly installed, an NMO self-seals its hole and will give you years of drip-free service provided you...
Don't forget to remove your antenna when driving under low underhangs! I got tires at a nearby Firestone store and they forgot to raise the service bay door high enough when they drove in and VOOOMP!...they peeled the dual-band antenna off the roof of my pickup! In the process the precision-drilled NMO mount hole got all bent up and it took a lot of patience and persuasion to get the hole smooth enough to readily accept the NMO mount again.
Good luck...hope you find what you need.
