Tuning too low will probably not damage the woofer but too much power if you try to play that low when it's unloading will damage the woofer. There's not a whole lot of need to tune below 30 hertz. Each woofer has it's own specs and those specs can be used to get a very reactive box to that specific woofer. Say 1 18 from X company may like 6 cubes and a ported box. But the next woofer from P company may like 4.5 cubes better and may like to be in a sealed enclosure. One may be suitable for bandpass while the other isnt. Etc. Each woofer will have a different response in it's driver specific enclosure.
Best bet is to read some design tutorials, and to once youve gained an undertanding, get a design program even if its just a box calculator and figure it out. You will have to know what to do with the TS parameters of a woofer if you decide to just download a free box calculator.
For basic design you can probably google a good enclosure volume for said woofer, then design off of some basic specs since you will know what to look for.
Or you can pay someone that already has all of this information and knows what they are doing to design you an enclosure and you can build it. Designs range usually from $20 and on up to style of enclosure and per each designer.
P.M. me if you would like a design.