It will take quite a bit.
1. Self-adhesive mirror film (can be bought in stores selling wallpaper). Window film will not work.
2. Sheet chipboard and the same hardboard.
3. Thin hose and sealant.
A ring is cut from the chipboard. Later I needed two rings. Otherwise, the beam will focus too far. The ring is cut out with a jigsaw.
A circle of hardboard is cut to the size of the ring.
The ring sticks to the hardboard. It is important to coat everything well with sealant. The design must be airtight and free of air.
On the side we make a hole and insert the hose.
And finally, pull the mirror film on top.
Then the air is pumped out of the housing and a spherical mirror is obtained. The hose is bent and clamped by a clothespin.
For this unit, it is desirable to make a stand.
Spar this thing be healthy.
It turned out to achieve good focus. The only thing that is bad is that this mirror cannot be directed to an arbitrary point. Only in the sun.
Then you can experiment with the diameter. For example, glue two or three strips of mirror film. Although already this size melts an aluminum can.
The process of manufacturing a solar reflector in the video: