cheapest way would be to take a small piece of 2x4 wood and a hammer and hammer the duct together as close to the trunk tiein as you can get, thereby choking the air down to the desired flow amount NOTE: this approach would save you approx. 5 bucks as you can buy a manual damper for bout 5 bones probably take bout 15 minutes to install. Times are hard and money is tight I understand so just tear the heck out of that duct line (smash it with a hammer) as required. You may be able to take out some internal frustration and also be able to save on your therapist bill?

Blocking the grille is a poor way to control air. If you can hear the air before, after you block the grille, you will really hear the air.

You could try to use a slower speed ob the blower motor.

If you are kinda handy with tools, and can use a pair of metal cutting sissors, you can make and install a damper for less than $10. If the duct is round, the last time I bought a damper it was about $3.50. If you have square duct, you can use several strips of metal placed like bars across the duct to make a damper. Temporarly, you could use some cardboard taped in place to block part of the duct. If you have flex duct, you need a short section of same size pipe, and the damper plate. You will cut the flex and splice in the section of pipe. tye the inside layer with a zip tie and tape, then tye the outer layer and zip tye and tape. The biggest obstacal is access to the correct duct. Do not do this in a main duct, only the branch. Do it as close to the beginning of the branch as possible.

This is a simple DIY project, easier than replacing a faucet washer, or changing a door lock.