Yes I machined a new extension and installed it. You can disassemble the system and figure out how much you need for the lenses you want to use. I would assume that they all use the same size front so I could make a piece for you or make and install it for you. Below is pictures of the extension. If you do disassemble it let me know the diameters.
I agree with Doug that the dome you have chosen is too small for that lens minimum focus distance.
You will need a 5" or larger dome and it would be best if you place it the proper distance away from the nodal point for that lens which is about 2" or 50mm. The front of the dome should be (the radius of the dome) + the nodal point from the front of the camera. So if you are using a 5" dome then the front of the dome would need to be 2.5 + 2 = 4.5" from the front of the camera. The virtual or apparent image is at 9.25" and the minimum focus distance for that lens is 7.8"
For the 100mm / 4" dome the virtual or apparent image is at 7.25" and the minimum focus distance for that lens is 7.8". So if you are using a 4" dome then the front of the dome would need to be 2 + 2 = 4" from the front of the camera and you will have to use a diopter,
If you are having problems with binding then you probably have the wrong zoom ring or lens clamp installed. attached you will see two zoom rings and two lens clamps. The zoom rings to the left are for smaller diameter lenses and have a taller bar on them then the ones on the right. the lens clamps on the right are different thicknesses also. Feel free to mix and mach as needed. When they are installed they should be snug but not binding. also don't forget that the rubber gripers for the clamps if needed.
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.