the rearmost U piece goes along the two frames on the sides and are welded together with 3-4 passes of the MIG. The pipes are joined on a 10cm length. I hope that gives it enough strength.

The reason i'm posting all those pictures is not showing off... I never fabricated anything that needs to take some dynamic loads and frankly, I have no idea how this would hold up to vibrations and bumps etc under some 25kg load.
I read too many blogs about luggage racks breaking up, and on the other hand i rear of a guy who made one of copper tubing for heating systems, filled with epoxy I believe, and toured Africa with it.

so I someone can tell me if it looks sturdy enough that would be great!

The whole thing is mounted on 3 points:
- top of the shocks
- center of the U frame that's the bike's rear now.

I'm still considering adding it an extra tube from those sticking out bolts where the fender is mounted to the U-frame. that could give some exra support to the bottom of the frames on the side. I'm not sure it's needed though. Tell me your opinion please!

Absolutely Dave.
The U frame is bolted on the shocks top mount and onto the frame where the original rear fender used to bolt to.  If those welds brake or if the U-bar bends and buckles it's done for. I'm sure I'd not want to have a decently sized wife sitting behind me!
The idea of adding some horizontal towards the front is great! I'll take a look at that! Thanks!

That’s a serious lookin rack.

Regarding your tire rub, you never want suspension set up such that a tire can rub.  The shocks have hard stops.  Those stops should limit travel so that the rear tire can never hit the fender.  If your tire can rub on the fender, you need to fix the problem.  Increasing preload on the springs may reduce the rub, but it doesn’t prevent the rub under all possible scenarios.

You need to fix the problem.  New stops to limit travel, different rear shocks that limit travel, a smaller diameter wheel/tire, raise the fender, etc.  The suspension system must limit travel so the tire can never rub.

DragBikeMike, I think i can limit the travel by new stops in the short time i still have left. Is it something i can do with the usual tools and a big vice? I foud some threads about pulling apart the shocks so that's not and issue, question is, can it be done with human muscle or will the spring put me through the garage wall?
(I even have a smallish spring compressor for cars somewhere but I doubt it will work here... can try though)

"Would a pair of longer Marauder shocks solve the problem?  Does it also mean that they stop higher up (not longer travel, just longer shock)?"

I'm not sure.  You need to know what the eye-to-eye distance is when the shock is fully compressed and against the stop.  Maybe Verslagen can comment on that.  The longer shock may or may not have more travel, and the eye-to-eye distance when fully compressed may of may not be greater.  You need more eye-to-eye distance when the shock is fully compressed.

FYI, I routinely put my rear shocks (Progressive 11.5") on the stops just riding by myself with no luggage.  I've tried heavier springs (90/130 & 115/155) and I still run the thing onto the stops when I hit a pothole or moderate irregularity.  I'm about 200 lbs with riding gear.

If you could get the shocks apart and remove the upper eyelet, you could possibly put spacers between the rubber stop and the upper eyelet.  I'm sure that's easier said than done.  This is the stop in question.

The stock shocks have a one-piece spring retainer and eyelet.  I have no idea how to get the thing off, and I certainly don't know any way to do it safely.

The retainer on the Progressive shock just slips out from under the eyelet once the spring has been compressed.