I was going to post this in a few weeks in the proper Mustang topic when I get a few more parts (it is still missing some parts of the left door and some other little pieces that I missed in my bricklink orders), but I might as well show a little now (and more in a few weeks). When I wanted to built the Mustang, the connectors mentioned above were already 2-3 euro a piece, since I wasn't going to pay 80-120 euro for 1 type of part (not to mention the 14L red flex axles) I looked for alternatives. My first idea was to make a black Mustang, but I dismissed that since 19L black flex axles weren't as available either (only used in "The Mission" set I think) and I didn't like the fact the wheels would be black, exterior black, white stripe and if I didn't alter the colour of the chassis, that being black too. I then looked at building the Mustang in white, but was shot down early due to some flex axles simply not existing in white. I looked at rigid hose options too for all colour schemes I came up with, but some lengths weren't available, meaning I would had to cut up hoses. I didn't want to do that, the only piece of Lego I used to cut up in the old days were... pneumatic tubes!
Loving the idea of the white mustang I had in my head I found a shop that had pneumatic tubing in white which I thought was original Lego tubing. I thought the Lego 9641 set had white tubing, but it's super light gray. I already ordered them when I found out, but when I recieved it, it was the exact same stuff as Lego, same thickness, same feel, same looks only it was white instead of the default blue, black or gray. This allowed me to built the Mustang in white.
It's firm and stays in place as long as you don't touch it
The black flex axles at the front are still 11L flex axles, didn't change those for tubing. For the blue hoses I wanted to use pneumatic tubing, for the added thickness, but the blue hose I ordered at the same shop as the white one, was a little transparant. Not looking like pure Lego I dismissed them for this model (will probably use them for some pneumatic stuff). I then used 3 11L rigid hoses, which look very well imo.
So I'd say as long as it looks like real Lego pneumatic tubing, go for it. It gives alot of room for more colour combinations than you ever could come up with only flex axles. And with more colour options available stuff like the 2-3 euro connector might not get caught in the crossfire of the flex axle price hike.
Also Sheepo, thank you very much for this awesome model, it was a great joy to build. Also thank you Jurgen for making the instructions with you.I can't wait to see your new model!