Hard to find plans, I found this:
From: T. Naumann
I don't know alot about guitar speaker cabinet design. I take it that open back cabinets take advantage of the drivers free air resonance, but how do manufacturers come up with the dimensions of sealed enclosures? It seems to me that the specs on the individual speakers don't matter much to designers if you can just buy four new speakers and drop them into your cabinet. Do they even try to figure that stuff out like HIFI loudspeaker designers do? For instance, if I had a Marshall 4x10, how could I be sure that four of your P10P's would sound right in it? Would it be worthwhile to try to custom design and build a sealed cabinet based on the T-S parameters of the P10P?
I sure wish I could state an answer as well as you posted the question. Unfortunately, I have never met, talked to, nor read any articles written by the original designers of speaker cabinets for guitar amps with regards to their philosophy of design, etc. Like most, I do have my own observations and opinions on them, though. No doubt, in a combo amp an open back enclosure is imperative to allow air to flow in and out for cooling the tubes and other electronic components. Having the back open also allows for a convenient storage space for the power cord, etc. The amp would sound louder overall at the expense of losing a little on the low end, the extra loudness being a plus for the marketing of the amp. Most combos I've seen are designed with just enough room for the speaker to be mounted with a symetrical border of baffle around it, while not interfering with any of the chassis components. Having been involved in production engineering as well as cost accounting, I can assure you there was also some thought given to how much wood and covering would be used for a given size cabinet using the standard sizes of material stock, while yielding the least amount of scrap. Another issue would be total shipping weight, since that would have to be factored in when arriving at a price for dealers. In a sealed enclosure, it generally takes more electrical power to get the same acoustic power as a similar sized open back. The sealed air in the cabinet behind the speaker acts as a resistance to cone movement, the smaller the enclosure, the more the resistance. Think of blowing up a small, tight balloon. You have to blow like heck to get it started compared to a large loose balloon. So, the bigger the box, the better, generally. Now think of the early '60s 'piggy back' amps. It wasn't long before players were standing them on end with the amp head sitting on top. Was it for better sound projection or because it looked cool? Of course, manufactures saw that, started making the 'stand-up' models, and made the cabinets larger but still used the same size speakers. Next is the question of designing the box to match the speakers technical specs. I've often wondered that myself, and have never really arrived at a good answer. Some recent amp manufactures, such as Dr. Z are using ported cabinets and are getting very high marks from reviewers. Nevertheless, these designers, like all guitar amp designers, are designing and building for a particular sound and performance as a guitar amp, not a HIFI speaker enclosure. In other words, the issue here is music 'sourcing', not music 'reproduction'. Your last question is one I don't think I'll ever be able to answer, even as a speaker manufacturer. I've had people call me and tell me "I gotta tell you, this is the worst speaker I've ever heard in this amp, way too dark". Yet, I've had others tell me the same speaker is the best they've ever had, and they've been trying for a long time to find the right speaker for that particular amp. To design a cabinet that matches the speakers technical specs will certainly smooth out its performance, make it a better mathematical model, so to speak, but on the other hand, much of the desireable guitar tone is in the comb filtering, cone breakup, and other distortions that occur when a speaker is taken to the edge of its design operating parameters. It's apparent this is why serious players buy, sell, and trade speakers, like amps, until they get the tone they are after.
Additional comments submitted by Joe Breher: Jim Marshall, the man who gave us the world's most emulated sealed box guitar amp speaker enclosure, was recently interviewed by a popular guitar magazine. In the interview, he stated that the dimensions for his cabinet were dictated by the smallest practical cabinet that would fit 4-12" speakers. Evidently, no thought whatsoever was given to the acoustical properties of the cabinet.
And that's from this site
http://www.webervst.com/sptalk.html .
. then just hammer together a pine box with enough room to mount what you have chosen for a speaker,... and plug it in....if, on the other hand, you do want it to look nice,... I can give you several alternatives for finishing it, and websites to go to for parts like handles, grill cloth etc....