How bad is the crack? Have you tried fitting it correctly in the position suggested above? Or are you reluctant to try due to the crack.
Rather than trying to find a tube to lengthen the spring arm. Why not re-enforce the plastic with some epoxy filler.
Specifically, my favourite, Milliput! It works like soft clay and dries harder than stone. A thin-ish layer of this over that area should be enough. It is super strong.
These are my repairs with it, I posted these elsewhere as a recommendation recently.
I used it to totally re-build the support arm for the lawnmower.Most of the bracket is missing under the yellow milliput area. It takes a lot of force from the push/pull of the lawnmower. So a super strong repair. All my glue efforts with the original bracket piece failed after a single use.
And sculpted a new catch on a dishwasher lid after it got smashed off.
I also used it to re-enforce cracked plastic around the screw holes as you can see.
So a thin layer similar to the last image is all I mean.
You can get the standard yellow version for about £2.80 on ebay.
It is a two part expoxy that comes in two sticks like plasticine.
Totally worth it. You don't need to use it all at once. You would need a tiny slither of each stick for a repair that size.