Another way would be to cut out one corner of your baffle 7-3/4" X 7-3/4". Then form two sides with two 14" pieces making kind of an "angle iron" and using the inside corner of the box to form the other two side of the tunnel.

The pieces you use inside should be [14"- the baffle thickness] because it all counts. Also, make sure the tunnel ends about 5" in front of the box back wall, so the inlet is unobstructed.

You could also use two 6" round vents or three 5" round vents, either 14" deep.

You can use a jigsaw to cut driver openings - I'll bet $1 that more people DIY speakers without a router than those who do use one. I assume from the way you phrased it you mean you're going to surface mount the drivers, not flush mount them, as in rebate them into the baffle.

If you intend to rebate them (flush mount) you're going to have to cut a rabbet around the jig saw hole with a piloted router bit anyway, so you ought to use a router to cut the hole in the first place. Otherwise, the irregular jig saw hole will cause the rabbeting bit to make an irregular recess, which will be kind of messy.