Sorry, I did not mean to suggest an extra adapter. My point was only that blocking the back works fine for me.
Regarding your illumination for testing, I suggest to do something like what I described above:
I stuck a piece of double-stick tape to a glass slide, used that to peel some scales off a moth wing, then looked at those using transmitted illumination.
Any old piece of glass would work for this, since it is behind the specimen and not in the optical path. A piece of window glass with a couple of layers of matte transparent tape on the back for a diffuser would work OK.
If you decide to keep the objective, then after you cut off the barrel you should be a lot more comfortable with getting light in. Remember that the objective is intended to work with light coming from around the imaging lens, reflected in by the outer barrel. By cutting off the outer barrel you will essentially just be exposing the normal BD illumination range of angles, plus a little, and with the potential to have asymmetric illumination.
The biggest challenge then will be managing to frame and focus without running the objective into your specimen. For that, I suggest calibrating the difference in focus distance between your high power objective and a lower power one with longer working distance. Then you can frame and focus safely, using the lower power objective, before mounting the higher power objective and adjusting for whatever the known difference in focus distance is. In my own case, I happen to know that a particular 10X objective focuses 6.30 mm different from my 40X NA 0.80 in its RMS adapter with bellows adjusted to be ideal for the 40X NA 0.80. So then I frame and focus with the 10X, back off by 6.30 mm, set a hard stop in my focusing mechanics, then back way off to change objectives, advance again to the hard stop, and voila, the 40X is close enough that all I have to do is tweak framing and focus before starting to shoot.
--Rik