This is a 3D printed version of the Sun Funnel that NASA published the instructions for building. You can find that here: https://eclipse2017.nasa.gov/make-sun-funnel

I wanted to customize the sun funnel to allow a larger projected image and use a larger eyepiece that I had on hand. The size of the projected image will depend on your telescope's focal length and the focal length of the eyepiece that you use. The document from NASA linked to above shows the math.

I'd recommend picking an eyepiece that makes a solar image that is 3-4" in diameter for this 6" diameter screen. That way there is some wiggle room allowing you to not have to continually track the sun but let it drift across the screen then reposition. Of course if you are using a tracking mount, then choose an eyepiece that fills the screen more 5-5.5" perhaps.

While it's possible to use super cheap eyepieces like Plossl's will work, having a bit wider FOV makes it easy to find and keep the sun in view longer for the same focal length.

My favorite short focal length eyepiece is the TMB Planetary series that run about $50.

Link: https://a.co/d/0k54NUh

You can find them on Amazon and elsewhere and they are available in many focal lengths between 2.5 and 9mm. I sized the eyepiece holder to match the 44mm outside diameter of these planetary eyepieces. You can use a thinner eyepiece like most Plossl's if you print off an adapter ring with an OD of 43.5mm and an ID that fits your eyepiece.

The NASA instructions suggest a screen made of rear-projection material and that works just fine. I've also found that white trashbag material works as well and is much easier to track down. Paper can also work if you can cut the edges so that the clamp can hold it in place without wrinkles.

You can use hose clamps to hold the screen and eyepiece in place, or you can print off the clamps here and use an M5 heat set insert and an M5 bolt to squeeze the clamp and hold it in place. The hose clamp is stronger especially on the eyepiece side, but the printed clamps are easy. I used CNCKitchen inserts with a 5.6mm hole before installing the insert. As long your inserts require a similar size hole you should be good to go.

I find it best to use an aperture mask to limit the heat into the system no matter what telescope I use. I highly recommend you do the same.

Use a high-heat filament in black. Black will improve the projected image since the contrast ratio will be better. However, black will get hot in the sun so something at PETG level or better is really required.

Just as a point of reference I use this primarily with an Edmunds Astroscan with a 450mm focal length. A 7mm eyepiece is about perfect for me.