This is a part that I designed inspired by the modular carriage by _MSA_ to make a stiffer z-axis as mine wobbles around as I don't have a double z-motor setup.
I haven't printed this mod yet (I'm on my way) but I hope that this helps to anyone with the same trouble as me.
I also added a back-support to put in the 4020 extrusions and make sure that they are squared with all the printer.
Except the base, all the parts are extracted from the _MSA_ design.
UPDATE 1 (19/08/2018):
I've printed the parts and screwed them and I had some issues with the back ones (specially with the ones that cut through the support of the 4020), I'll try to fix it ASAP. Also the tensioner part broke tightening the screws. I glued it back with acetone but I'll modify the desing to avoid this issue. PS: I've printed the part and tested it, in the section below I'll show the issues that I had
UPDATE 2 (23/08/2018):
I've uploaded a new reinforced part(left-gantry-modded-v2.1) that should get rid of all the bendings and algo has inserted the nuts for a better general fit, as soon as I print it there will be photos, so stay tuned!