All of these can be customized to fit what you want.
It seems what you’re after will be a head that mounts tubes for different lens lengths. If you’re head won’t do this, then you will have to change out the head. Most of these tube mounts have the head mating part with no ability to change it. In all likelihood, you’ll have to change the head out to get a different lens tube.
Mirror size – if all of your mirrors are 18mm, but the beam sees them at a 45 degree angle. So the view of the beam is an 18mm tall by 12.7mm width (18 * Sin(45)).
If you tube produces a 6mm beam, then 12.7 - 6 = 6.3 / 2 = 3.3mm available for beam alignment on each side of center. Meaning that if you’re out over 3.3mm, part of the beam will likely be cut off.
Just keep this in mind, your beam size and mirrors maybe different sizes.. so you can plug in the numbers if you need to.
I think anything you find to do this, if you’re current one doesn’t allow it, will require a head transplant.
Make sure you get what fits your machine… many of these are not specific to any machine.
There are a number of on-line calculators for lens length and related depth of field (dof) and spot size.
Keep in touch as things change, you can update us on your solution.
Good luck
