1st time builder

hi its my first time building a custom box.  I have some FG experience so i know how to lay it but i've never made a mold or anything.  I want a custom box to go in the back of an s-10 blazer complete from one side to the other- a ten on each side with my cap and amp mounted in the middle.  i can calculate dimensions for box volume and all myself but i dont want any gaps anywhere.  What is the best way to make a mold that will fit to the back of my back seats and go a little way down the sidepanels of the rear compartment?  I'm gonna do a custom front of the box so I just need the back and sides molded to fit?  Please help.   Donnie

Lots of 3M masking tape, 2-3" wide. At least two layers that overlap. Then user spray glue on the vertical areas (so the mat sticks) and spray release on the horizontal areas (so the mat doesn't stick). Then add mat and resin, etc., etc.

the FG wont penetrate the masking tape?  I dont want it to go through and ruin any of my upholstery or side panels.  What about foil?  I've heard of that but not real certain on how it is used.  Donnie

The resin won't go through the tape but use at least two layers and overlap the seams to avoid leak through. Foil always works but it can tear, get pinholes in it and doesn't stick to vertical surfaces very well.

once i get a couple of layers on and the FG is cured, will it be safe to pull it out of the truck to finish? and should it just come out easily?Donnie

Yes, you can finish it outside of the vehicle. It should come out relatively easy. The vertical areas may have bonded with the tape but since the tape can peel off, so can the mold.

would it hurt not to put some spray release on the horizontal areas?  the tape will just stick to the FG right?Donnie

You need the mat to stay in place on the vertical areas. If you used spray release it would just slide off. You can try taping the mat to the top of the area and let the mat hang down. Then could use spray release. You'd need the base piece to have a slight incline so gravity would make it lay flat.

yah i understand about putting spray adhesive on vertical areas but i mean on the horizontal areas, is it necessary to put spray release?  If i dont use any, the resin will just stick to the tape on the bottom right?Donnie

That's correct. You don't need to use spray release. It will just bond to the tape and then you peel the tape off the base.