Like Don, I just make sure to always set the brake firmly enough that it is very obvious it is set. If you can't easily set it so the car will only barely move, then there is something wrong with the linkage or rear brakes (contaminated linings, perhaps).
But if you really want to add a warning, I would look at mounting a "micro switch" or limit stop switch under the floor, so the end of the handbrake lever presses on the switch when the brake is released. Here is one example:
https://www.acehardwaresuperstore.com/2246-specialty-switches/45329.html
Under the floor like that, it probably needs some protection from the elements. Fully sealed switches are available, but are kind of expensive. There's an Altoids tin laying here that looks like it would make a good enclosure, after adding a slot for the lever to protrude through.
Once you've got a switch, the next question is what to wire it to. Personally, I would prefer to avoid any visible modifications, like an extra light on the dash. One approach might be to use some diodes to make the turn signal indicator do double-duty (constant on for brake warning, flashing for turn warning). The method I would probably use would be an audible alert like
https://www.mouser.com/ProductDetail/Mall...jUOuU5ynAFwk%3d
perhaps with a diode and big capacitor in series (so it only sounds briefly as you turn the key on).