Edit: I have made the chamfers smaller and added draft angles to the slots for the 1-2-3 blocks to attempt to avoid fitment issues, while still allowing good grip.

I have one of those cheap suction vises that they sell online and Harbor Freight and whatever, that doesn't like to actually stick to anything, and doesn't have much weight either. I designed this base to hold 1-2-3 bocks so that there's real weight. You can just unscrew the top from the suction vise and screw it onto this one.

This only has 0.3mm clearance on each side of the 1-2-3 blocks. In my test prints this gave a really nice, close fit. The blocks didn't slide around at all, despite the one side being so open.

I measured the threads pitch at approximately 2.5mm. The threads on the design are 2 1/4-10 UNS threads, which is the closest preset Fusion 360 had.

I was able to print this without supports, just bridging the ceiling of the block cavities. While that succeeded, I think it caused the warping you see in the pictures (discussed below) which won't allow the 123 blocks to fit.

Issues
The ball interface on the real vise base only has three contact surfaces, rectangles that protrude from the concave surface. This increases the grabbing force (also being metal and not slick plastic). This version with just a basic concave cutout doesn't like to lock down as easily. It does function, but I'd recommend a wrench for tightening and loosening like this one: https://www.thingiverse.com/thing:3851692. This design could probably be improved in that area. For now I've just attached some 600-grit sandpaper to the surface to add some extra grip.

The BIGGEST issue is when printing. You can see in the pictures the top of be walls bend inward before they reach the ceiling of the block cavities. I think this is caused from the high cooling happening when bridging the ceiling. It doesn't happen if you have a low number of walls, like 2. I printed this with lots of walls, so those sides were entirely walls. Supports might fix this, but I'm unsure, and that's why this is still marked WIP.

SV01

No

No

0.3

20

lots of walls and thick top/bottom for durability to allow clamping of the base to a table