If the plunger can't fit, you can also use a knuckle to gently self massage your patella tendon. You can go up and down or across the tendon. Might feel a little grisly, but as long as you don't force anything, you can't hurt yourself.
Do that seated as well with the leg relaxed.
Also seated, you can use the index and thumb of both hands to pinch and try to raise the patella tendon in an effort to decompress the area. If you can get a good grip, try and hold for 5 minutes to get a good release.
You can also while seated use the same pincer grip as above to gently grab and lift the actual patella in an effort to raise and decompress the patella.
Same 5 minute hold if you can do it.
There won't be a lot of range of motion while lifting either structure, but the idea is really to try to unweight either the patella or the tendon as its being compressed down by the tight quadraceps.
Again, won't cure anything, but may give symptomatic relief if done gently. Almost like if the pressure was being released, if that makes sense.
Babolat APD/RPM Blast 18G @ 53#.