Hi James,
First, really nice work! The 4' width is going to give multiple garters plenty of freedom in each section.
I recently made a new enclosure, pictures below, and have some ideas that you (and others) might be able to modify to suit your needs. First, I should probably say that it's not finished in order to disguise my poor carpentry skills and make people think that great and wonderful things are soon to come.
I have an 8" x 17" (21 x 43cm) heat pad underneath the 3/4" (1.9cm) floor and outside of the enclosure. It's along part of the right wall (under the silver thermometer and in front of the round log with the round hole). All walls are 3/4" (1.9cm) thick, and the heat pad makes the hot spot a little warmer than I'd like. The astroturf helps to diffuse a bit of the heat and lowers the temp by almost 1C, which currently keeps the hot spot between 88 and 90F (31-32C). I still need to get a dimmer for it because I'd like to have the hotspot around 85 (29.5) or so. The cool end stays at about 21C.
The wires to my full spectrum lighting run from the ceiling, down behind the vertical 1" x 3" (2.5 x 7.5cm) malmite strip in the front. I cut a 2.5cm disc out of the floor and ran the wires out through the hole. Then I re-inserted the wooden disk, so it fits snugly against the wires. Then I put some tape above and below this disk to make sure that it doesn't get pushed through the floor somehow. It's actually very snug, though.
My plexiglass doors in the front have handles on each side, and both slide sideways to open. They can be completely removed. I've considered cutting a hole in the top of the enclosure and using a router to recess another plexiglass window/door from above. This would give even better access, but it might also let more heat out. I haven't decided whether to do that or not. Anyway, the fluorescent light fixture is just far enough from the front so that the plexiglass can slide (with a bit of resistance), but not far enough away that a snake could get between it and the glass. The same small space exists behind that vertical 2.5 x 7.5cm malmite strip in the front. On the far right and left sides (next to the handles), the plexiglass extends about 2" (5cm) behind the vertical trim.
There's also a small, white shelf in the back corner, but the snakes haven't used it much yet... just to occasionally investigate the ceiling. I think I might attach some old shed snake skins to hide the unfinished edge on that back shelf, or I might put a piece of horizontal trim and fill it with wood chips. That would be more difficult to clean, though.
Also remember that your snakes will be spending a lot of time wherever your heat source is, so put the heat source where you would like to see the snakes. Finally, there is an electric power strip that you can see near the front. The wires come out of the enclosure and plug right into the power strip, which has a timer regulating the lights and heat.
The perimeter is roughly 36" x 36" x 22" x 35", and it is 12" tall (.91m x .91m x .56m x .89m by .3m tall). The longest internal dimesion is 50" (1.25 m) from right to left. It's not square, so that it doesn't stick out too far into the room.
Hope this helps to make yours even better.
Rick