Internal Focusing (IF) lenses start out bigger than non-IF (the latter can be considerably more compact when focused to infinity, for storage).

IF lenses also change focal length when focused closer. The Canon 100mm USM that I use is more like a 70mm at full 1:1. Not that you notice this in the field.

And, the CURRENT Tamron SP 90mm f/2.8 VC USD does not extend. It is Internal Focusing too. (The more compact, crop-only Tamron SP 60mm f/2 is also IF.)

The previous version of Tamron SP 90mm (not VC or USD) is not IF. (This version can still be found new, too)

The CURRENT Sigma 105mm f/2.8 DG OS HSM is also IF and does not extend. Previous version (not "OS") might have been non-IF.... I don't know.

I can't find a review that compares the Tammy and Siggy with the Micro-Nikkor 105.... but this shows the two earlier version, third party non-IF lenses next to the Canon 100mm IF lens:

https://www.the-digital-picture.com/Reviews/Sigma-105mm-f-2.8-EX-DG-Macro-Lens-Review.aspx

Notice that when they are at full 1:1 magnification, the non-IF and IF lenses are almost identical in length.

This review of the CURRENT Sigma 105mm OS version shows it alongside the two Canon 100mm and the current Nikkor 105mm VR (all four of which are IF):

https://www.the-digital-picture.com/Reviews/Sigma-105mm-f-2.8-EX-DG-OS-HSM-Macro-Lens-Review.aspx

In this last review, there's no illustration view of those four lenses shown at closest focus, simply because there's no difference. All are IF, none of them extend.

As to any disappointment about the impracticality of
reversing the Tamron 90 ... by what science did you
come to believe that was a good idea anywho. I'd bet
it was the "collective received wisdom" of the Online
Experts ... such as abound hereabouts.

Think on this: The Tamron is modern 1:1 macro lens.
You can safely assume it has internally shifting lens
elements that accomplish "Close Range Correction"
[thaz official Nikon jargon, aka CRC]. If you reverse
mount the lens, you defy the designed-in corrections
of the CRC system. Reverse mounting is necessary
for ordinary, unit focusing, conservatively designed,
general purpose lenses when we apply those lenses
to the abnormal usage of having the subject-to-lens
distance shorter than the sensor-to-lens distance ...
which is the reverse of general use, and thus calls
for reversing ordinary lenses. "Tit-for-tat" simple :-)

If "A Little Knowledge is a Dangerous Thing", forums
full of Online Experts are PREMIER showcases of that.
Enjoy your lens ... in its intended application !

And only take hardware advice by users who actually
USE the hardware in question. OK ? I USE a Tamron
90 SP, in Maxxum/Sony-A mount ... so it CANNOT be
reverse mounted. To get "more magnification" [sic]
than the built-in 1:1, I just swap the FF body for an
APS-C body. Same 24MP, plus a 1.5X crop factor :-)

After all, "1:1" is a just a conversational fiction, ever
since we switched from film to digital sensors. I love
to mention that, cuz it brings out all those indignant
Online Experts with their geeky arguments ! And as
this is a macro lens thread, I know they're reading it.

.

As I said I need the shutter to be open continuously for up to several hours at a time. Sensor heating doesn't allow this.
This has nothing to do with photographically other than me thinking I might be able to use a camera and lens combination to do it (plus a TV screen). And it would work other than the sensor heating.
I was trying to use existing equipment but no go on that. I suspect a several thousand dollar stereo microscope would work but don't want to spend this money and then find out it wasn't a good idea possibly for as yet unknown issues.