Assuming by "load" you mean impedance (given that you're dealing with Ohms, load is current which would be Amps, which isn't important), it would be a problem if the speaker impedance added up to less than that of the amp output, it would cause the current flow to increase to the speakers but drop the voltage, so your amp will overheat. That said though, you're not going to find an amp that has a speaker output with impedance greater than 16 Ohms, they just don't make them.
Although if you meant that the other way round, whether the speaker impedance can exceed that of the output jack (which now reading your post properly it looks like that's what you meant), then no, it isn't so much of a problem, for instance if you had 2 16 Ohm speakers wired in series to equal 32 Ohms, it wouldn't damage anything, however again you wouldn't get all your juice to the speakers. Generally you want to try and match impedances for optimum performance.
As far as wood goes, I wasn't altogether arsed, my dad just got some treated plywood (treated with what? beats me!). Mine also has a back on it, all your bass response escapes without one I find. And yes, a regular guitar jack will work, although I would reccomend a proper amp jack 'cause it would be sturdier.
