Actually ive been doing some research on one of those DIY trimmers aswell, and came to it being the best to combine 2 things for the trimming procedure.
My guess (without having read it all, sorry for that), that u used the grill from a manufacturer of the trimboxes, right ? Would be my first choice Anyways... My idea kind of came as i was trimming the edges on my lawn... It used those plastic wires, but i know u can get those in steel too which keeps a good straigh line, and if not those, then theres propably something else, metalic kind of wire which doesnt "bend down"...
Ok... So lets say we have our steel bucket, or round container or what ever, that the grill is attacked to... What we need is the trimming placed inside this... My idea was that the bottom got cut out, so that the trim could pass through. To get it down there, we need something to suck it down, same thing with the wire for rotation... Is possible, find a high grade electric moter that u have the ability to connect things to... as a ventilation fan... The fan doesnt need to be that big, as long as it creates a suck to the grill, out the bottom of the bucket. The motor should have some sort of metal bar all the way to the grill, maybe even through it (just through) so it keeps a steady rotation... 15 cm ventilation fan on the bar aswell, and in the very top, the metal wires... Those are the ones that cuts the bud, as the bud gets slightly pulled down with the fan...
So the perfect thing for this would be to catch all the trimming in a bag, which could be mounted on a table of some sort... The bag could be anything as long as the holes aint too big, and it still "breathes". Make it long, tie it in the end, connect the top to the bucket and voila... A trimmer
Im still searching for the right parts to make this wonder machine, tho i havent found it all yet Got the bucket, ventilation fan and might have a motor soon too hehe. Maybe i could use the one from the ventilation, but ill have to do some testing on its strength.
But please do drop in a few ideas of your own if anyone got any... Maybe we could combine some things
I am not sure if you are joking or serious here. Other than using string line you are simply describing what I built. I tried string line early on and didn't like, other's do, to each his own. Good luck with your project.