Darin...... first things first. Everybody has a diffeent "school" of thought on this and no one in particular is right. Here's my humble opinion....
No "door" for visibility is needed. There may be one on a standard oven, but I have yet to see one on anything else but a lab-oven in industrial use. Just another point where you would encounter heat loss. Even if I were to put one on, it wouldn't be made of plexi as it would melt the first time you brought the whole thing up to temperature. Also, sheetrock would not be a recommendation I would give anywhere near that oven. Steel inside (usually a nice thick gauge),insulation suited for the aplpication inbetween and steel on the outside (not nearly as thick as the internal metal). Anything having to do with any part of organics (the paper on sheetrock for example) would not have longevity in this atmosphere and quickly degrade. All steel contruction ovens with a good grade of insulation between the panels is industry standard. The seal around the doors is usually a fiberglass reinforced cording and latches on the tops of the doors (which USUALLY swing open barn-door style) are of the "C" hasp variety. Think positive locking tumbler type. I'm sure you've seen them before.
Now... with that out of the way, let's continue on with your inside chamber configurations. As most in here would tell you.... the taller "hanging" type of oven would be the way to go. Inside dimensions of 20"W X 20"D X 40"Tall would be my suggestion. I say this for two reasons. First being that it's always easier to hang a part on a hook than it is to lay it width wise in an oven. Especially one of that length. One could fashion a simple hook and even hang more than one part in there at a time making the efficiency of the oven more viable. Let's say you wanted to hang one part in and start it's cure cycle. Go right ahead. Now you've just coated another part and wish to cure that as well. Go right ahead on that one too. Keep going until you have the oven full. Once the first part is done, take it out and replace with yet another one. See where I'm going with this? This is called "revolving batch coating" in the industry. A constant flow of worker and oven effieciency. If you tried that with a "lay down" type of oven... no doubt you'd have the worst nightmare ever trying to get the part in the back when you have 3 others in front of it. Here's the second reason, with a twist.....I'd also tend to make this oven 52" tall incase you wanted to build some sort of tray to put small parts on and bake something totally different in while all this is going on. Simply weld or bolt in metal angles every 6" evenly on both sides of the oven and cut 20" square (because squares won't roll around on you) steel bars to fit. A few in a row can support a tray while one or two below can support whatever you are hanging. Change part size? No problem. Just move the bars up and down to suit your needs. It's that simple. Want to do a lot of trays worth of parts all at once? Slide in a couple of bars on every level and you can have a bakery rack style chamber to cure in. It's certainly more useful to you in this manner than if it were a lot wider than taller.
But then again.... that's just my opinion
..... Russ