Excess fleece

are the sides going to be shown? if they are there is a little more work involved. You have to staple the fleece down to the box on the sides as cloe to the edge as possible. The generously apply your resin. Then you have to go back and lay fiberglass matting along that same crease but on the inside of the box for added strength. Once everything is hardened and strong, go back with a vary course sanding disc or grinder and grind off the extra fleece with reisin in it to make that side of the box flat again.

If the side is not going to be shown, just staple the fleece down and resin it. dont worry abotu the way it looks.

There might be a better way of doing it but thats how I do it.

Cougar beat me to it!  Definately, rabbiting the edges is the fix before its an issue at all.  Also, as Nouseforaname said, bury the excess in the rabbited edge...so...

Add this to your mental data-base:  A box we built only required a small section of FG to curve deep into the box to feature a face of an amplifier and a 8 inch sub...together.  Kinda hard to explain the look.  Anyways, it was pointless none the less to fiberglass the whole side of the box just to make this indentation.  So, a rectangular hole was cut to allow fitment.  Then about 1 inch around each side of the rectangle hole I use a router to cut a "trench" as if it were a moat for the hole.  I used a

router bit like this oneto cut only 3/8 inch deep.  I stapled the cloth as close to the inside of this router cut "trench" as possible.  When I cut the excess polyester fleece cloth, I lay the excess that passes the staples down into the "trench".  I applied the resin to the cloth.  After the cloth is saturated, I pour the rest of the resin into the trench to fill it, along with the excess cloth.  When this is final it finishes off flush with the original surface, easier to smooth out overall which relieves you of the useless voids you have to sand, sand, sand....

Its supposed to get hard.  But, its initially only going to get as hard as the type and thickness of fleece cloth you used....(i.e. if you use thin cloth, it will only soak up so much resin....if you use thick fleece it will soak up much more and be harder).  But, it sounds as if your may be already beyond that point...so, when you add layers of FG mat or FG woven cloth with more FG resin it gets even more hard if you have mixed the resin as "hot" as it should be for the temperature and humidity in your area.  Ultimately, the "Shade Tree" fabricator's test is: "If you can stand on it without it breaking, its strong enough!", and there is also the thumb test....  Read this: https://www.the12volt.com/installbay/forum_posts.asp?tid=42117&PN=1.  Jason of CarAudioHelp.com sent the link in this post to Jeff / Velocity with good explanation.  Jason has been a great source of basic and advanced information that can take you from rookie to....well, as far as your capabilities can go. 

I / we would love to see the photos of your first try.  I wont be laughing as others may, because I remember my "back then" jobs.  lol  But, the pics would be a great influence for other beginners to see and learn from another beginner.

Oh boy, that "Roller" topic again!  You sand between layers of FG mat/resin because when it cures a bi-product is formed as wax which does not allow good adhesion to the next layer, thats been the argument.  This wax is caused by "cheaper" resin.  Use marine grade to avoid this, but dont dwell on it, what you have will work just fine.  I dont always follow this depending on the situation, because of past posts Ive been involved in.... Ill stay reserved on this issue.  But, as far as the FG roller, you more than likely going to be ordering it.  Its only important when you need one. 

So, your using FG mat?  Did you read where everyone tears the FG mat into peices?  Apply the resin on your "pre-cured" fleece, then slap on the ripped FG mat, apply more resin on the mat to wet it out...doing each piece like this.  When "wetting" or "saturating"  the FG mat, your making sure resin is pressed into the mat with your brush as well as making sure there are no excessive bubbles.  If you have done this, you have no real reason to require a FG roller.

i havent matted yet, i will tomorrow,

I guess ill have to stick to Bondo Resin, i found a place that has Rage Gold Body Filler so i guess im good on the bodyfiller, but my resin isnt so good. Im gonna buy the black decker sander, but which one should i use the 4-in-1 below or the smaller one below that, Its a ten dollar difference between the two.

Sonata's got it right.....use something that will knock the surface down if your going to do it at all.  The sanders you pictured dont have much movement. 

Yeah, your good with the Rage filler.  But to smooth the Rage, those sanders may work ok if you dont have an air type DA.