I had the idea of trying an arm myself yesterday too, coincidently.
A different approach though...my idea was with a big emphasis on 'trying' w/o expecting I will really build a good arm.
The way I came to that was I was tempted to try an elderly quite good cart, but the arms in fashion now won't suit such carts too well from compliance, mostly.
Well not the arms I would like to try...and to buy a really expensive arm just to run a cart in it that doesn't run near optimum from compliance point of view means that part of the arm's cost is kind of wasted not suiting the cart really perfectly..so my guess.
The whole thing started off with resonance-testing records for carts, the idea of the elderly but obviously good MC I had in mind
and then try with quite a simple self made unipivot to get some arm together to match the compliance of that cart more or less spot on
via test-record and software.
No expecting to build a great arm in first instance..just to get one build that matches this cart perfectly from the compliance point of view.
Then check if the arm is any good at all, compare it putting the cart into a good arm that will suit it's compliance quite not so well.
Either my selfmade approach is a total looser then despite perfect compliance match, then I will stop at that point.
(not having invested much time into it really)
...or my arm is doing somewhat that far ok that it seems worth try refining it a bit or think further in that direction.
So, as a thought for a start, I would start with the choice of an MC system and try to make an arm that suits the compliance of that MC perfectly.
If your choice is a modern MC system, your proffessional present competition, also s/h will be rather tough, obviously..
The base-idea of my aproach is to have the arm-cart compliance spot-on on my side
and hope to compensate a certain part of my not-really-existant competence to build a good arm.
In other words I don't think I can match any professional arm-maker, but am curious about how far or close I will be able to place my throw..
In short: Fix your choice to a specific system and compliance and I'm sure you will have a bit of a head-start with your arm.
I do like the idea !
I'll do a 12"..if it sounds shit, at least it will look impressive..
Uni..like Simon said, I can't imagine making a cardanic on my own in my more or less just-for-fun approach..