How to get a decent workout on a mid sized, flat parking lot? I'm looking for a way to have a bicycle based workout in the middle of the COVID-19 quarantine.

I do not own a roller trainer of any type, nor materials good enough to make one myself. In my country there is a full(ish) lockdown and I cannot go out for training.

I live on a building complex that has a mid-sized parking lot but it is almost completely flat.

I'm searching for a way to increase resistance to the bike to get a meaningfull training out of a 40-60 ride.

The first idea thar I have is to put a rubber band to depreess the rear brake lever to make it act as a drag brake. (disk brake) (Would it damage or excesively wear the brakes pad/rotor?)

Another idea is to put some sort of "fins" into the rear tire to make it generate air drag (Maybe attached to the spokes perpendicular to direction of travel).

The goal is to get a decent workout at a moderate speed, the parking lot is not big enough to accelerate a lot, and does not provide a loop path, (I'd be like making a 'C' shape going back and forth.) Also, as it is a parking lot, I do not want to go full speed to avoid hitting or scaring a neighbor who may be walking to/from their car ans suddenly pops up in front of me.

The fins idea could also be adapted to a simple stand that lifts up the rear tire to make it trainer like.

Any additional ideas on how to get a workout are appreciated. I'd also appreciate anecdotal information if you wave opted for such an approach sometime. 

I'm DIY oriented and Currently I have with me a bike with street tires, and a bike with mtb tires. A spare worn out wheelset and some tools, and I cannot go out shoping for parts, materials nor extra tools.

EDIT: Here I cannot buy online parts nor other goods than food or medicine. There is simply no such comerce stablished in this country. Any online buying I've done involves shipping to a P.O. Box and paying a second carrier to bring it from USA to my country. Currently such service is not active as only health/food/journalism workers are authorized to move on the street.

I did read some applicable ideas on this answer: https://bicycles.stackexchange.com/a/61837/2177
Consider constructing a parachute similar to those worn by sprinters: https://www.amazon.com/SKLZ-Speed-Chute-Resistance-Acceleration/dp/B002CLK21C

This would generate significant drag at moderate speeds.  Just be sure not to get it tangled up in the rear wheel!  Perhaps install an emergency release to avoid getting tangled yourself.  It's been discussed here: https://www.bikeforums.net/33-road-bike-racing/273066-anyone-ever-use-sprinter-s-parachute-added-resistance.html

Another option is to wear bulky clothes that would act as a parachute/sail.  I often achieve this effect unintentionally with rain gear.  Throw your bike into high gear and grind away!