Would a double thick front panel for a sub enclosure be best?

A quality sealed enclosure should have a double-baffle (what you described). A double-baffle doesn't add any airspace so you could just build the box according to rockford's specs and then simply add a second layer of MDF to the front. I imagine it would change the displacement slightly but not by a degree that would be important. It's worth noting that sealed enclosures don't need to be all that exact...in fact, most subs will provide a range of specs such as: .9-1.25 (or something along those line). For example, I have a TC sounds sub which recommends 1.25-2.0; so again, being slightly off really isn't a big deal. Good job on deciding to build your own box...the quality of prefab boxes keeps going down and down.