Is there anything like spring steel hold-down probes to help with SMT assembly?
Nothing.  No springs, no clips, no glue.

Clean all solder off all the pads for your part.
Apply a small blob of solder to one pad on the board.
Move your part into position with a pair of tweezers, then heat the pin and pad so the solder melts.
Press the part down so it sits flat.
Remove the heat, let the solder cool while holding the part in place.
Solder all the other pins.  The first one will hold things in place just fine - at least as well as the glue and a darned sight better than any wiggly mechanical construction.


I missed your mention of &quot;free space&quot; assembly of parts.
This known as &quot;dead bug&quot; assembly.  For that kind of thing, it is better to stick at least some of the larger parts to something larger.
A piece of protoboard is good.  I've even used a scrap of cardboard from a cereal box at times.
Glue the larger part(s) down with some thing fast drying, then use fine wire to connect the smaller parts.
Magnet wire or wire wrapping wire are good for the interconnections.  I've also used telephone wire at times.
Here's one I did a few weeks ago to try out a circuit:

It looks rather ugly because I had to go back and burn some of the glue off of it.  The glue was effectively a short circuit between the gate and the drain and source of the JFET transistor in the middle of the mess.
That hacked up &quot;prototype&quot; proved the circuit so that I could have a bunch of PCBs made.
They look like this:


It cost about 70€ to have 12 PCBs made, and I didn't want to go straight to from idea to PCB if it didn't work.