...so you have replaced the front standard with an L-plate? sorry can I bother you for a bit more detail?
I initially started by shortening the Actus base rail to the length of the sliding carriage assembly and then attaching a piece of .250" thick aluminum angle to the remaining piece of base rail and in turn attaching the Actus lens panel to that via a pair of M6 screws (the second of which screws into the opposite side of the aluminum angle and then only partially through it, hence isn't visible in these photos. I would have remade the angle piece so both of the screws weren't visible, but I was too lazy to bother because almost all of the cutting and shaping was done manually using a hacksaw and files, thus took some time...):
I subsequently added some "speed holes" to both legs of the aluminum angle and trimmed the width of its base leg slightly to reduce its weight and painted it with wrinkle-finish black paint:
And then after I gained some experience laminating multiple thin sheets of carbon fiber into a single, thicker sheet as part of another camera project, I decided to shorten the other aluminum leg and then restored it to its original length by attaching a piece of 5 mm thick carbon fiber sheet, which is how the camera appears in the first photo I posted to this thread.
Be warned that since I was working mostly by hand here, getting the aluminum angle aligned sufficiently parallel to the sensor in both the horizontal and vertical planes to avoid introducing an unwanted tilt or swing (which, of course, was the point of undertaking this modification in the first place!) was done via trial-and-error and took quite a bit of time. But by working carefully, it
can be done, as I eventually proved!
Of course, if I had access to a mill, precisely and accurately machining this part from an aluminum billet would be an easy task, not to mention one that could be done in a fraction of the time, but one can only work with the resources available to them, eh?
That said, while this approach worked well for me and my purposes (I rarely ever need or use tilt or swing, which is why the inability to lock down the front standard was so frustrating!), I am not necessarily recommending it to others. And if / when I ever decide to sell the Actus, all I need to do to return it to its original form is replace the shortened base rail with an OEM Cambo rail (which cost just over $100 last time I checked, hence the reason I was willing to tackle this project, because I knew I wouldn't permanently destroy its market value as a result.)
Other Actus owners have had some success in this respect by simply tightening the tilt and swing mechanisms, which makes them more difficult to rotate, both voluntarily and involuntarily. I tried this, but ultimately decided to replace the front standard entirely because it also saved something like 13 ounces of weight, which is noticeable after carrying the camera many miles during an outing.
. I've never tried the 25mm. There are two versions: the DA one that only covers 33mm x 44mm, and the older D-FA, which covers 645. You'd want the D-FA to allow movements. Alas, neither has an aperture ring. You can buy an adapter that allows controlling the aperture (it's the one Ed mentioned -- e.g., from Fotodiox). However, that device is meant to mount the lens to a GFX camera, not to a Cambo Actus. I believe the Actus Pentax 645 mount board is for the "normal" Pentax lenses that have aperture. 
