Kingsnake makes it very easy to post pictures but you need to have them hosted online somewhere. You cannot just upload them from your hard drive and onto a forum post. Fortunately KS also offers a gallery in case you do not already have a photo hosting site you use. Here are two links that will be helpful:
Tools & Toys
Kingsnake's Gallery
I was thinking of building the cages out of plywood covering them with the stuff they use in showers(sorry I don't know the terms).
You'll have to be a bit more specific. Do you mean the tile-board shower surround material that comes in 4x8 sheets? Or the similar fiberglass reinforced panel (FRP)? Or do you mean the vinyl or polyethylene membranes that go over walls and underneath tile? Or you do mean the paintable moisture barriers?
I wanted to use Ceramic Heat Emitter to heat the cages, is that a good way to heat them and how many watts do I need.
I like CHE's and feel that large snakes in particular enjoy basking underneath them. They do get very hot and some sort of barrier which prevents the snake from coming in contact is very important. Regarding wattage, that depends on a few factors including the height of the cage which you did not specify, the amount and placement of ventilation in the cage and the coldest temperature of the room where the cage will be placed. There is no forumula so you'll likely have to experiment.
Also will a 4'x2'x36" be big enought for full grown boas.
Large enough, but I'm unsure if the 36" is the height or the depth (front to back) of the cage. 36" is an uncomfortable depth for a cage unless it is very tall and you can lean into it. I would re-think it if you meant 36" as the depth of the cage.
If 36" is the height then you'll be fine, especially if using a CHE. My only concern would be if the room where the cages will be is cool in the winter. In a larger cage like that you may end up getting too much of a thermal gradiant. As much as I like taller cages for boas I would be a bit concerned about heating a cage like this in a cool room. But there are ways around this. You could wrap the cage in Reflectix during the winter and provide a lot of thermal mass and get a very nice distribution of heat.
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Currently keeping:
6.10 Gonyosoma oxycephala (Javan, mixed colors)
1.1 Philodryas baroni
1.0 Rhodesian Ridgeback