Finding a material transparent to 26GHz radio waves for radome protecting a radar rangefinder probe
1. Any dielectric
A dielectric slab has a reflection from both faces, which interact and can cancel the transmitted wave. One way to manage this is to keep the radome very thin (see below), perhaps less than 1/10 wavelength.
The other way is that a dielectric will transmit the wave effectively if the slab is half a wavelength thick. There will still be losses in the material itself, but the reflection loss will be minimum there.
This is a diagram I found of the transmission through a slab(Rutgers):

At 26 GHz, half a wavelength is 5.8 mm in free space. In the dielectric light is is slower by the square root of the relative permittivity of the medium.  For a common low-loss plastic like polypropylene or polyethylene, with Er of 2.3, the half-wave thickness is 2.3 mm. So a 2.3 mm thick radome should provide minimum loss (and a 1.2 mm radome provides maximum loss)
2. Quartz.
A strong, transparent radome for 26 GHz can be made of quartz glass (quartz fibres in epoxy) less than 1 mm thick. Quartz has a lower dielectric constant than glass, and lower loss. It's also more expensive and harder to buy.
If this isn't strong enough for your application, a sandwich of (say) 0.5 mm quartz, 2.5 mm engineering foam and quartz again.
For a single patch antenna or a small group of them, the radome won't need to be very big. You might make up a front panel that's say 3 mm glass fibre for most of the panel, and then switch to 0.5 mm quartz just for the window in front of the antenna. Trial and error using a sheet of aluminum foil with a hole in it, to find the required window size.