Consider the alternative of a vertical rather than horizontal windlass. The vertical windlass has the advantage that it doesn't care which side the chain comes in from. They are as strong running in one direction as the other. Accordingly, mounted on the center line, they can service
either bow roller. Moreover, a vertical windlass is not very reliant on the weight of the chain to hold the chain into the gypsy. By discharging the chain forward, the chain is in contact with the gypsy for 180º verses 90º on a horizontal axis gypsy.
Given the foregoing, you could fairly easily mount a "shelf" to the aft portion of the chain locker, sufficiently high to accomodate the drive motor
between the bottom of the locker and the underside of the shelf. (A simple removable cover can be fabricated to isolate the drive from the rest of the chain locker.)
For example, the Maxwell
VW 1000 is the vertical equivalent of the Lewmar you're considering and needs only about 10-11" between the underside of the proposed shelf and the bottom of the locker. The shelf itself could be made of a couple of layers of 1/2" plywood
glassed together in place and to the sides and back walls of the chain locker, high enough to allow a fair lead from the bow roller to the chain gypsy when the windlass is mounted. Of course, the back of the locker cover would have to be cut out to allow the gypsy/capstan to protrude, but a cover for the windlass could be fabricated using a plastic bucket as a mold
We utilized a similar arrangement to the foregoing on our boat and it has worked very well for both bow rollers although one does, occassionally, need to kick the chain pile forward to get it out of the way.