This is not meant as a flame or a personal attack but this thread shows the danger of reading cases from sources other than the primary ones used by attorneys. Ensor has been distinguished many times. A search on Westlaw or Lexis shows this. Also Ensor is not about secure encasement, its about concealment.
Secondly Ensor is not based on the current statute which was amended in part because of Ensor.
As for the cite to the Ashley v. State, 619, So.2d 294 (Fla.1993), The language that was cited to was the Court's discussion of a conflict case, Amaya v. State, 580 So.2d 885 Fla.App. 2 Dist.,1991, where again the issue was concealment, and ready accessibility not secure encasement. There is no language in the Amaya decision or the FL SC's opinion that the gun in Amaya was securely encased.
Please do not cite to or quote cases that have not been verified as current and not overturned. It does a disservice to all.
A gun in a car is not automatically considered a concealed weapon in FL. Dorelus v. State, 747 So.2d 368 (Fla. 1999).
There are three important factors to this analysis for the OP.
1. Is it concealed? This will always be tricky and fact specific. I assume a gun in my car is concealed because it makes the rest easy to comply with. The open carry prohibition does not apply if you are in a car.
2. Is it securely encased? This is very easy in FL based on the statute. In a snapped holster or something I have to lift the lid on is good enough. If it is securely encased I do not have to worry about it being immediately accessible for use, because securely encased means it isn't. Other not immediately ready and accessible cases discuss whether the gun is loaded and where the ammo is in relation to the gun, usually because the gun was not securely encased. These cases are confusing and contradictory. I am not going to do a full analysis of that issue without pay. Bottom line, doesn't matter how quick I can get it out to use. If you use the statute's definition to determine securely encased it is not immediately accessible.
3. Is it on the person? Using this provision to carry in a car without a license, it cannot be on you, or I would argue touching you. Where can it be? The statutes are pretty clear about Glove boxes and consoles. There is also the Gemmill case where a box with a lid on it (containing a gun) was wedged between the driver and passenger seat.
To the OP, is under the seat ok? Even securely encased you get into trouble for on you. There is no reported case I am aware of which says that securely encased firearm under your seat is on you. Not being your attorney and not giving specific legal advice on this board I cannot answer the question as to what might be determined in your case if stopped. For discussion purposes, under the seat is most likely not on you. From a SD standpoint, can you get to it there as readily as in your closed center console or glove box, if not why put it there? Under a seat is just a bad idea for a lot of reasons. The is it on you question is the least of them.
From an attorney who does car accidents- NEVER HAVE ANYTHING, ESPECIALLY A FIREARM, EVEN IN A HOLSTER, WHERE IT WILL FLY AROUND YOUR CAR IN A WRECK.
EDIT The pre 1980 cases cited in this thread are dangerous. The changes in 790 since then make any pre 80's case of questionable value and they must be carefully considered.