I reused my uncompletely formed sheet today and it worked quite well!
This is the first try when I put it on the vac too early and it did not have enough sag. The left side should be a 90° angle and some of the corners should be sharper. You can see some burn marks the heatgun left when I tried shaping it with hot air on the vac. It did not heat the ABS well enough to form.
Drilled more holes around the mold which worked way better. For the next one, I will add even more holes and/or dremel tiny grooves.
Reheated the badly shaped form and allowed it to sag way more than before. Some of the corners looked still a big too sturdy, but it worked out. Mind you, I used the exact same mold and placement of the sheet again, so the corners went on the corners again (if that makes sense, heh).
2nd time it worked way better, but at the 90° angle it could still need more holes or a tiny grove to get a perfect angle. I am not sure the reused sheet would have worked out that well if it was used for a different mold than the first one but for this one, I see no downside to reusing it. Could be that it will degrade a little with every reuse, but one more should be fine.
My oven is a simple 22mm OSB box covered in tin foil and some kind of stone grill inside (sans the stone) which works pretty well. The wooden frame with the ABS clamped between just goes atop the box with not much wood exposed to direct heat, so no problem there either (I kept a fire extinguisher around anyway). The cost of the whole setup was probably about 10€, so below the cost of a 3mm 50x30cm sheet of ABS. The MDF mold releases very well from the plastic, no sticking at all there.
Will carry on with this project when I come back from vacation. Next step: Need a case that is about 10-15cm long that this case can slide into. One doenside of the ABS tho might be that I can not simply epoxy stuff onto it like it would be with wood.