Are there any spare parts on board??? Ikelite users probably use the same C Clip?? if not you might get buy with placing a proper fitting washer on the shaft and then twisting some fine wire in the grove where the C clip is supposed to be. I hope you have the rubber / vinyl cap also. this may work.
Also if you don't need the button or another button remove the button, remove the spring reinsert the button and either use the wire or tape solution you suggested.
A vacuum system would also help but it may pull the button in and activate that function on the camera.
best of luck, I am sure you will have spare C clips on the next trip.
Attached you will see my solution. A little different than yours but same idea. I made a adapter plate to fit between the housing and the handle. Two screws from the plate into the housing one one side and two post that fit into the handle and one screw that goes through the handle, the adapter plate and connects to the housing. I attached Glued a extension on the shutter release so that you can trip the shutter while still holding on to the handle.
In regards to the tight fit of the UnderWater Camera Stuff 3.8" Precision dome that is fitting tight, make sure that your using generic silicone lube on the O-rings NOT OLYMPUS grease. If you still find that the fit is too tight you might try something just a little smaller that would stretch more and thus have less thickness. We use a 3mm x 74mm o-ring so you could certainly use a 3x73 or maybe a 3x72. The only concern would be that it doesn't leak. I prefer a tight fit as opposed to leaking.
Here are a couple of suggestions. Caution the parts are made out of brass and then chrome plated so there somewhat fragile and soft.
Put two nuts on inside the housing locking them together and allow as much room as possible between the nuts and housing. Turn the inter-most nut to loosen the threaded bolt.
Put the nut back on without the o-ring and see if you can get it locked on to the threaded bolt and use that to turn it out. You may want to put a small washer in the nut to take up some space between the nut and bolt head.
Put a odd ball nut inside that will not properly thread on to the bolt and try to turn the threaded bolt out.
Once you get the threaded bolt loose it should turn freely.
I find it very comforting and I am sure others would agree to be able to look at the flashing green led once I am in the water, and several times during the dive. My counts are floods = zero, leaks = less than 6, water damaged equipment = zero. On the other hand the flashing red led is not so comforting but it is better than damaged equipment.
If you elect to purchase a system without electronics PLEASE make sure that you have a FUNCTIONAL moisture / leak / water detector in your housing. Some of the lower end housings do not have this installed as standard equipment.
We are currently working on a solution similar to what Chris requested. I hope to have a prototype working within the next three months and will announce it here on wetpixel when it is ready for market. We are hoping to keep the cost down to be in line with a 180 degree viewfinder such as the Aquaview or something similar in the $1000 range.