I'd take a can of urethane foam (sold as insulation
and caulking), sold under the name "Great Stuff" here in the US, and just shoot the foam into the outer 50mm pipe, using the longest nozzle to get it down a bit. The foam will expand to fill, seal, and block the pipe without crushing it or your fuel lines, and you'll have a solid plug in there, which will be adhered so well that it would take a LOT of water pressure to move it out.
You can dig it out, or mash it up, using a dowel or screwdriver at a later date if you need to get it out again. But it sounds like you need a semi-permanent plug that will remain in place until the fuel lines need to be changed.
The "proper" way to penetrate a bulkhead would be to have a metal shop fabricate a plate with three pipe nipples welded into it. Fuel lines to be attached (screwed on, banjo clamps, whatever) on either side, after the plate was welded into the engine room bulkhead. Fiberglass
...no welding, but the same idea.
Not a terribly difficult long-term fix to "do it right".