I'll make a post of the better way to mount it once I return to Oslo. Perhaps later today.
If you have 37-52 step rings, K-5, and BR2/K-2, you should by the way have all the pieces you need. Unscrew the rear section, put the step ring the to act as a guider, add the K-5 and tighten. Can't remember whether the final F-mount will be provided by BR2 or K-2, that depends on the thread gender on the free end of the K-5, but one of them will fit and provide the bayonet for attaching to camera or bellows.
At least I think the K-5 was the required spacer, but it might have been the K-4. You soon enough discover which one is the correct part when you start juggling around the bits and pieces.
As an exercise in patience, I recommend sitting down at a work table filled with step rings, adapters, and the odd parts from you scrap bins, and make something out of the parts. One always finds new or unexpected solutions even to problems one didn't realise existed previously