I am trying to build one now. I don't have a clue, really, but have been making some progress.
"Free Hot Tub", delivered to my house.
Now the fun part. It was tough for me to take on this project, as I value my time, but my girlfriend wanted me to proceed.
So I built a deck, at first it was 8x8, but after looking at the tub sitting on it, the deck changed to 8x12. The tub is a VitaSpa 5x6 with 6 jets.
I have determined the height from the deck to the bottom of the lips as being 25 1/2". So I am framing a wall around it. Then I will be shimming underneath it. I intend to use scraps of wood and foam. Once the tub is secured, I will be needing a spapack.
Here is what I started with regarding the framing.
Now looking like this.
I intend to build a box off to the side of my spa, install my pack in there and run the pipes through the exterior and into the old cabinet for the connection.
I also intend to convert it from air controls to electronic. Orginal pump is 1hp, but the new pump will be ordered at 1.5hp. Blower attached to pack. Buying a filter system that will need plumbed in within the attached box, so that I can lift out the filter more easily. The box MAYBE a bad idea, but I think that it will help curb the noise of the pump running. Again, it may be a mistake not to fit it into the existing compartment.
I will have spent about $1,000 on the spa to refurb it, but it will be brand new in that aspect. So not too bad a deal. The deck I would have had to build no matter which tub would be used. Also, having to pay an electrician to run the power and install the anti- electrocution module thingy is going to cost additional monies. But the spa shell is in great shape and the cover is as well. I figure their value to be about equal to the cost of the refurb.
I want to see it running with flush before I enclose the sides. The connections may have leaks.
I see your project as being even tougher. Mine is about as difficult as I could imagine anyone wishing to take on. I noticed that the compartment inside the spa had been wet for some time, as the pump was rusted badly. I would be concerned about using a metal frame. The humidity, contained within, would degrade the metal over time. I suspect these spas are naturally moist and warm underneath. Pick your material wisely.
Also, mine will not be portable once finished, but rather stuck in place. I don't intend to move anytime soon. BUT....if when you make yours, make certain that you build a skid underneath your frame so the integrity of the frame is not compromised during a transport.
I offer my mess in response to your request for ideas. Good luck with your project. I hope mine goes smoothly. Alot to learn about spa packs, water treatment, etc. Good website.