Probably have to start with some idea of acceptable tip loading - 1M is over a milliamp, can you go lower-Z in pursuit of BW or will DC accuracy force you to accept worse?
Might think about a capacitor divider and a regular 1M/7pF probe to get better edges but still Heisenberg wants you to look at the effects of trying to measure, on what's being measured.
If the measurement is single ended, I would start the search with off-the-shelf 100:1 probes, e.g. PMK PHV1000, 50 MOhm/7.5 pF input impedance and up to 4 kV peak voltage rating. I don't hear a preference for DIY in the original post.