OK, jus wanted to clear sum stuff up that i've seen people saying on the site...i'm no expert by any means but i've made a good bit of fiberglass work.
now then. to get the outragous shape that is always desired, you have to think it up first. know what you want to put wherever you want to put it and at the angles you want to do it at....simple? if you want to have a custom box that will curve to the cars needs or hide over in a corner or w/e, take tape. simple masking tape works great, tape off the area and use trashbags for bigger areas, just to make sure there will be no excess resin dripping anywhere because it will not come off a freshly painted car. to do it that way, just lay some resin down, make sure its mixed with the hardner correctly or this will cause alot of anger. take the resin and slop it all over the area you want to do, i've found that fiberglass matt you can buy from your local auto store is alot stronger than the cloth, but not as smooth for a final finish...anyways, take that, go past the area you want to have the base for, make sure you have at least one layer of matting down, get it soaked in the resin as well, jus dabb it down with a paintbrush. wait however long it takes (depending on the weather) and when its dry your going to pull up the tape and the base part of whatever you made. pulll the tape off (suggest using gloves) and let that side cure, it is a chemical reaction but still needs air. from there, make your speaker ring, monitor ring, w/e, and position it to whever you want it to be, get some wodden dowels and hot glue (works great) trim the dowels to whatever length they need to be to get your ring at the right angle, and make sure its rather solid. NOW THEN. take fleece (works great and is cheap) and cover the front of the scructure to get that crazy look, if you dont like it you may need to add certain things to get the shape you really want. then slop on the resin. now you have your basic structure. from there after that drys just keep sloppin up the thing like its your girlfreinds face and add small peices of matting; using a paintbrush to get out any bubbles which will cause cracks, do that at least 3 layers problably more pending on how strong you need it. just a general rule i've been telling people that i've taught, make sure its strong enough to have sex on, if not it will crack under usual wear and tear and vibrations. then use whatever type of body filler you'd like to use, smooth it out, paint it or cover it or whatever. there ya go. usually if its a sub box, the weight of the thing on top of all ur components in there will keep it in place. if not u can reach in through a speaker hole and bolt it down.