If you used a rubber bushing, Vaseline would work fine. If you're using a "bellows" type, you don't want to use lubricant as it is made to be fastened to the pushrod. This is one example of a bellows type:
https://www.towerhobbies.com/product...PRB286072.html
Here's one place you can get clear plastic sheets for the radio box cover:
Polycarbonate Plastic Sheet, 11"W X 14"L, 2 Sheets (micromark.com)
They also carry a wide variety of other things, including tools, that you might need for your next build
I run 50% nitro fuel in my engines. I tried 20% helicopter fuel in a Nova Rossi .21 and found it wasn't enough to get it to run.
Zippkitts are a good boat kit. They are, however, very different from the Dumas kit you built. If you are looking at a thunderboat kit, they are twice as at least 50% longer and roughly twice as wide as the Pak kit. They are also made of all birch ply, just like the skin panels of the Pak. This means a high-quality epoxy is a must, not what you can get from a hobby shop. One problem is that, due to a shortage of plywood, all of their kits are out of stock at the moment.