Make a Custom door or use Original for Sensor
If you cannot access a spare back door for your camera, then you can easily fabricate one out of aluminium sheet or other materials. The new door does not have to be the same as the original. You are not trying to make it exactly the same and it does not need to seat into the light seal trenches. The APS-C sensor pretty much seals up the film box so you only need to make the new door fit neatly and be dust proof. Think outside the square and you will come up with ideas that will suit you. I purchased a sheet of aluminium “Alu Stucco Sheet 0.6mm” from the hardware store and fabricated a new door in just over one hour.

New aluminium camera door. The top sits against the original door’s edge and the bottom overlaps the camera to the depth you require.
This material is light and pliable and easy to bend into shape with hand tools. A simple side hinge is made by bending the sheet into a half round and sliding a piece of wire into the camera for a hinge.

On the inside stick a line of light seal foam around the perimeter of the film door edges. Insert an old film canister into the camera and close this door onto it. Then pop rivet the door to the film canister and reopen it. You can see I have packed the bottom of the canister with tape so it is a nice tight fit and this holds the new door nicely in place.
Now we have a new door that will allow the Sony Nex to be mounted to it. The door is easy to remove allowing you to slip the film door back on quickly. Once you mount the Nex to the new door you may want to add some internal stiffeners to beef it up a bit.

Position sensor and heat shield in centre and mark edges with tape.

Cut tape away from the film area. Measure down from light seal edge to sensor copper surround.

Remove the sensor and reinstall the camera door. Mark out the position of the cut out for the sensor. Note that the top of this custom door fits neatly against the edge of the film back light seal lip where as the bottom of it just overlaps to the depth you require.

The film door is cutout and installed over the sensor. I just drilled the corners and hit the edges with a sharp chisel. You can extend the camera base flush to the new back by fitting a custom base out of aluminium box section.
The above process is the same if you were using an original spare film door. But with them you need to remove the film pressure plate and level out or remove a few rivets. You might also find you need to panel beat out the area around the sensor so it fits nicely in the cutout location.
There would be other materials you could use for a custom door. Some suggestions would be various metals like Copper, Aluminium, Stainless, Galvanised metal sheet, Steel sheet, Perspex (which can be heated in an oven (or use a hot air gun) and moulded into shape). Or have one designed and made in 3D printing. They can either be painted or you can purchase the stick on camera leather look finishes.
Finishing Under the Camera Body
When the Sony Nex is mounted to the SLR camera it will hang below the SLR. On the Nikon FE and FE2 I use an old MD11 Motordrive which gives me a good handle and means I can mount the camera on a tripod or attach it to my Metz flash frame. You may not have a motor drive and some cameras have motor drives that protrude out the back so making them unsuitable in this arrangement. So a solution is to use say some box section aluminium that is the same width as your camera base. Fit a through bolt that is flush at the bottom and screws into your existing tripod mount. Then fit another tripod mount nut to the base. Hey you can take the one out of the Sony Nex and use it for this purpose.
It would be advisable to fit this base extension and then pop rivet the custom door to it for additional strength. Then the custom door is held in place by the Rewind connected to the film canister and the Base extension which is bolted to the camera tripod mount. Fabricate and fit this before you fit the Sony Nex to the new door.
So there is the problem solved for a suitable custom door for your old collectable SLR camera. Fitting an “Electric Film” unit to your SLR is now possible with no damage at all to the originality of your camera. It can swap back to being an original film camera in a minute or so. All you Leica and Nikon F3 owners have run out of excuses now!
I hope this gives you a few ideas for your camera project to digital. Feel free to add some comments and further ideas below.